GEM has registers to configure the Tx SRAM segments distribution across
queues. The reset value is apprioriate (even spread) but we need to
care if/when number of active queues is modified (or if we inherited
unevenly initialised hardware from bootloader).

To distribute segments, we take as input the number of queues
(bp->num_queues) and the number of segments (found inside DCFG6).
Its output is a number of segments for each queue, formatted as
powers-of-two (eg 2 for queue 0 means it has 2^2=4 segments).

As the distribution logic is quite complex (at least its initial
versions had bugs), it is kunit-tested and those tests live at the end
of macb_main.c. To test:

⟩ env --unset=CROSS_COMPILE make ARCH=um mrproper
⟩ env --unset=CROSS_COMPILE ./tools/testing/kunit/kunit.py run \
        --kconfig_add CONFIG_NET=y \
        --kconfig_add CONFIG_COMMON_CLK=y \
        --kconfig_add CONFIG_MACB=y 'macb*'

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
---
 drivers/net/ethernet/cadence/Kconfig     |   6 ++
 drivers/net/ethernet/cadence/macb.h      |   5 ++
 drivers/net/ethernet/cadence/macb_main.c | 135 +++++++++++++++++++++++++++++++
 3 files changed, 146 insertions(+)

diff --git a/drivers/net/ethernet/cadence/Kconfig b/drivers/net/ethernet/cadence/Kconfig
index 5b2a461dfd28..3ae7123352f5 100644
--- a/drivers/net/ethernet/cadence/Kconfig
+++ b/drivers/net/ethernet/cadence/Kconfig
@@ -51,4 +51,10 @@ config MACB_PCI
 	  To compile this driver as a module, choose M here: the module
 	  will be called macb_pci.
 
+config MACB_KUNIT_TEST
+	bool "KUnit test for MACB" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	depends on MACB
+	default KUNIT_ALL_TESTS
+
 endif # NET_VENDOR_CADENCE
diff --git a/drivers/net/ethernet/cadence/macb.h b/drivers/net/ethernet/cadence/macb.h
index 30fa65e2bdf2..81fdd17b34db 100644
--- a/drivers/net/ethernet/cadence/macb.h
+++ b/drivers/net/ethernet/cadence/macb.h
@@ -193,6 +193,9 @@
 #define GEM_TXBDCTRL	0x04cc /* TX Buffer Descriptor control register */
 #define GEM_RXBDCTRL	0x04d0 /* RX Buffer Descriptor control register */
 
+#define GEM_TXQSEGALLOC_LOWER	0x05A0 /* Tx queue segment allocation (low) */
+#define GEM_TXQSEGALLOC_UPPER	0x05A4 /* Tx queue segment allocation (high) */
+
 /* Screener Type 2 match registers */
 #define GEM_SCRT2		0x540
 
@@ -543,6 +546,8 @@
 #define GEM_PBUF_CUTTHRU_SIZE			1
 #define GEM_DAW64_OFFSET			23
 #define GEM_DAW64_SIZE				1
+#define GEM_SEGMENTS_BIT_SIZE_OFFSET		16
+#define GEM_SEGMENTS_BIT_SIZE_SIZE		3
 
 /* Bitfields in DCFG8. */
 #define GEM_T1SCR_OFFSET			24
diff --git a/drivers/net/ethernet/cadence/macb_main.c b/drivers/net/ethernet/cadence/macb_main.c
index bac83a2b4c4d..022577756eab 100644
--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c
@@ -2751,6 +2751,59 @@ static u32 macb_dbw(struct macb *bp)
 	}
 }
 
+/*
+ * Distribute evenly available segments across queues. The computation is
+ * complex because (1) segments are counted in powers of two and (2) a queue
+ * can only use up to 8 segments. There are four types of cases:
+ *  - Sharing all segments equally is doable. Take num_queues=4 and
+ *    num_segments=16. Each queue will get 2^2=4 segments.
+ *  - Sharing all segments is doable. Take num_queues=5 and num_segments=16.
+ *    Three queues will get 2^2=4 segments and two will get 2^1=2 segments.
+ *  - Sharing all segments is not doable because not enough queues are
+ *    available. Take num_queues=1 and num_segments=16; queue 0 can only have 8
+ *    segments.
+ *  - Sharing all segments is not doable because not enough segments are
+ *    available. Take num_queues=4 and num_segments=2.
+ *
+ * We start by computing the power each queue will have. For num_queues=5 and
+ * num_segments=16, each queue will have at least 2^1 segments. That leaves us
+ * with remaining_segments=6. If we increase the power for a queue, we get a
+ * delta of 2 (2^2-2^1). The first three queues will therefore be advantaged
+ * and each have 2^2 segments. The remaining 2 queues will only have 2^1
+ * segments.
+ */
+static u64 gem_sram_distribute_segments(unsigned int num_queues,
+					unsigned int num_segments)
+{
+	unsigned int pow, remaining_segments, i;
+	unsigned int num_advantaged_queues = 0;
+	u64 val = 0;
+
+	/* pow=0 for all queues. ilog2(0) is dangerous. */
+	if (num_queues >= num_segments)
+		return 0;
+
+	pow = min(ilog2(num_segments / num_queues), 3);
+	remaining_segments = num_segments - num_queues * (1U << pow);
+
+	/*
+	 * We can only distribute remaining segments if (1) there are remaining
+	 * segments and (2) we did not reach the max segments per queue (2^3).
+	 */
+	if (remaining_segments != 0 && pow != 3) {
+		unsigned int delta = (1U << (pow + 1)) - (1U << pow);
+
+		num_advantaged_queues = remaining_segments / delta;
+	}
+
+	for (i = 0; i < num_advantaged_queues; i++)
+		val |= ((pow + 1) & 0b11) << (i * 4);
+	for (i = num_advantaged_queues; i < num_queues; i++)
+		val |= (pow & 0b11) << (i * 4);
+
+	return val;
+}
+
 /* Configure the receive DMA engine
  * - use the correct receive buffer size
  * - set best burst length for DMA operations
@@ -2832,6 +2885,19 @@ static void macb_init_hw(struct macb *bp)
 	if (bp->caps & MACB_CAPS_JUMBO)
 		bp->rx_frm_len_mask = MACB_RX_JFRMLEN_MASK;
 
+	/*
+	 * Distribute Tx SRAM segments evenly based on active number of queues.
+	 */
+	if (macb_is_gem(bp)) {
+		unsigned int num_segments;
+		u64 val;
+
+		num_segments = 1U << GEM_BFEXT(SEGMENTS_BIT_SIZE, gem_readl(bp, DCFG6));
+		val = gem_sram_distribute_segments(bp->num_queues, num_segments);
+		gem_writel(bp, TXQSEGALLOC_LOWER, val);
+		gem_writel(bp, TXQSEGALLOC_UPPER, val >> 32);
+	}
+
 	macb_configure_dma(bp);
 
 	/* Enable RX partial store and forward and set watermark */
@@ -6031,3 +6097,72 @@ MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("Cadence MACB/GEM Ethernet driver");
 MODULE_AUTHOR("Haavard Skinnemoen (Atmel)");
 MODULE_ALIAS("platform:macb");
+
+#ifdef CONFIG_MACB_KUNIT_TEST
+#include <kunit/test.h>
+
+struct macb_sram_segments_case {
+	unsigned int num_queues, num_segments;
+};
+
+static void macb_sram_segments_test(struct kunit *test)
+{
+	const struct macb_sram_segments_case *p = test->param_value;
+	u64 val = gem_sram_distribute_segments(p->num_queues, p->num_segments);
+	unsigned int i, sum_segments = 0, max_assigned_segments;
+	unsigned int num_queues = min(p->num_queues, p->num_segments);
+
+	for (i = 0; i < num_queues; i++) {
+		unsigned int q_segments = (val >> (i * 4)) & 0b11;
+
+		q_segments = 1U << q_segments;
+		sum_segments += q_segments;
+		KUNIT_ASSERT_GT_MSG(test, q_segments, 0, "queue %d, val %#llx", i, val);
+	}
+
+	for (i = num_queues; i < 16; i++) {
+		unsigned int pow = (val >> (i * 4)) & 0b11;
+
+		KUNIT_ASSERT_EQ_MSG(test, pow, 0, "queue %d, val %#llx", i, val);
+	}
+
+	max_assigned_segments = min(p->num_segments, 8 * p->num_queues);
+	KUNIT_ASSERT_EQ_MSG(test, sum_segments, max_assigned_segments, "val %#llx", val);
+}
+
+struct macb_sram_segments_case macb_sram_segments_cases[] = {
+	/* num_segments can only be powers of two. */
+	{ .num_queues = 4,  .num_segments = 2 },
+	{ .num_queues = 1,  .num_segments = 16 },
+	{ .num_queues = 4,  .num_segments = 16 },
+	{ .num_queues = 5,  .num_segments = 16 },
+	{ .num_queues = 15, .num_segments = 16 },
+	{ .num_queues = 16, .num_segments = 16 },
+};
+
+static void macb_sram_segments_case_desc(struct macb_sram_segments_case *t, char *desc)
+{
+	u64 val = gem_sram_distribute_segments(t->num_queues, t->num_segments);
+
+	snprintf(desc, KUNIT_PARAM_DESC_SIZE,
+		 "num_queues=%d num_segments=%d TXQSEGALLOC=%#llx",
+		 t->num_queues, t->num_segments, val);
+}
+
+KUNIT_ARRAY_PARAM(macb_sram_segments,
+		  macb_sram_segments_cases,
+		  macb_sram_segments_case_desc);
+
+static struct kunit_case macb_test_cases[] = {
+	KUNIT_CASE_PARAM(macb_sram_segments_test, macb_sram_segments_gen_params),
+	{}
+};
+
+static struct kunit_suite macb_test_suite = {
+	.name = "macb",
+	.test_cases = macb_test_cases,
+};
+
+kunit_test_suite(macb_test_suite);
+
+#endif

-- 
2.53.0

On Thu, 05 Mar 2026 18:20:15 +0100 Théo Lebrun wrote:
>  MODULE_DESCRIPTION("Cadence MACB/GEM Ethernet driver");
>  MODULE_AUTHOR("Haavard Skinnemoen (Atmel)");
>  MODULE_ALIAS("platform:macb");

these macros should be at the end of the file
you should probably move the test to a new file

> +#ifdef CONFIG_MACB_KUNIT_TEST
> +#include <kunit/test.h>
> +
> +struct macb_sram_segments_case {
> +	unsigned int num_queues, num_segments;
> +};