From nobody Mon Feb 9 01:29:35 2026 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 23BCC27057D for ; Thu, 23 Oct 2025 21:26:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761254813; cv=none; b=bCUPmTJdG4UK9iNCtnT7XyjIyI1+uKvL3uorhUhxgXdRao0/ndGfarK0jFB3GhqidOmwWwTpcnqCFkgxy1OzAKYksN445KDh+rkBEhOGJwooGNvCqDNYRk+7LqlxoAJ81GV1ljpJap14CyyDI1Nfboa3ng+G4NcCxW96sNEv9/k= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761254813; c=relaxed/simple; bh=F32XcE9uZaLeIAugKoH/5kYhwOSDtMq6/Znu/zqWWHs=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=IUXe85JPj6P+PLggaR7K2aPXAea8bTzVy9pb9ev5ZJNHVr4eM2YGnV6jFynqD5j5DU+g71Z5VwwwA/z2H8hXhzsP48TdPin7nJ7ZcY+lrCLl7s0PfCatZQUVi2nCNfcfp+Tm7JdVKSTBGOPOo523r9NvDjgV2qcN06SbR8B3Pb8= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Zh63lbpy; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Zh63lbpy" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1761254811; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=zAmyMgT9xxjxNHcsyNCmwAK+5c4B/XqUwz4L+lbngcI=; b=Zh63lbpyMW97rBl0POvF+9OtMcdwjlj4yBboYtfX6RYw8mADyKMSc+0hmuF3HiLbqBATZE vYEVdj1hQ8lltE+zCAGNhqhmTAsrNVkWUVQibgK6pGbHDP55b+vtG89TPrkPJA0nYM0uRb Mvc09lU2/oeyW2UhwBVLtxhu0/uuvu0= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-130-R200hXuuNxel_l9nS5WQ3w-1; Thu, 23 Oct 2025 17:26:47 -0400 X-MC-Unique: R200hXuuNxel_l9nS5WQ3w-1 X-Mimecast-MFC-AGG-ID: R200hXuuNxel_l9nS5WQ3w_1761254803 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 43E461975AF7; Thu, 23 Oct 2025 21:26:43 +0000 (UTC) Received: from chopper.lan (unknown [10.22.64.235]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 5178230002FC; Thu, 23 Oct 2025 21:26:39 +0000 (UTC) From: Lyude Paul To: dri-devel@lists.freedesktop.org, rust-for-linux@vger.kernel.org, Alice Ryhl , Daniel Almeida , Danilo Krummrich , linux-kernel@vger.kernel.org Cc: David Airlie , Simona Vetter , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?UTF-8?q?Bj=C3=B6rn=20Roy=20Baron?= , Benno Lossin , Andreas Hindborg , Trevor Gross , Asahi Lina Subject: [PATCH v5 6/8] rust: drm: gem: Introduce shmem::SGTable Date: Thu, 23 Oct 2025 17:22:08 -0400 Message-ID: <20251023212540.1141999-7-lyude@redhat.com> In-Reply-To: <20251023212540.1141999-1-lyude@redhat.com> References: <20251023212540.1141999-1-lyude@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Content-Type: text/plain; charset="utf-8" Currently we expose the ability to retrieve an SGTable for an shmem gem object using gem::shmem::Object::::sg_table(). However, this only gives us a borrowed reference. This being said - retrieving an SGTable is a fallible operation, and as such it's reasonable that a driver may want to hold onto an SGTable for longer then a reference would allow in order to avoid having to deal with fallibility every time they want to access the SGTable. One such driver with this usecase is the Asahi driver. So to support this, let's introduce shmem::SGTable - which both holds a pointer to the SGTable and a reference to its respective GEM object in order to keep the GEM object alive for as long as the shmem::SGTable. The type can be used identically to a normal SGTable. Signed-off-by: Lyude Paul --- V3: * Rename OwnedSGTable to shmem::SGTable. Since the current version of the SGTable abstractions now has a `Owned` and `Borrowed` variant, I think renaming this to shmem::SGTable makes things less confusing. We do however, keep the name of owned_sg_table() as-is. V4: * Clarify safety comments for SGTable to explain why the object is thread-safe. * Rename from SGTableRef to SGTable rust/kernel/drm/gem/shmem.rs | 50 ++++++++++++++++++++++++++++++++++++ 1 file changed, 50 insertions(+) diff --git a/rust/kernel/drm/gem/shmem.rs b/rust/kernel/drm/gem/shmem.rs index 45b95d60a3ec7..21ccb6c1824be 100644 --- a/rust/kernel/drm/gem/shmem.rs +++ b/rust/kernel/drm/gem/shmem.rs @@ -173,6 +173,25 @@ pub fn sg_table(&self) -> Result<&scatterlist::SGTable= > { // pointer to a scatterlist Ok(unsafe { scatterlist::SGTable::from_raw(sgt) }) } + + /// Creates (if necessary) and returns an owned reference to a scatter= -gather table of DMA pages + /// for this object. + /// + /// This is the same as [`sg_table`](Self::sg_table), except that it i= nstead returns an + /// [`shmem::SGTable`] which holds a reference to the associated gem o= bject, instead of a + /// reference to an [`scatterlist::SGTable`]. + /// + /// This will pin the object in memory. + /// + /// [`shmem::SGTable`]: SGTable + pub fn owned_sg_table(&self) -> Result> { + Ok(SGTable { + sgt: self.sg_table()?.into(), + // INVARIANT: We take an owned refcount to `self` here, ensuri= ng that `sgt` remains + // valid for as long as this `SGTable`. + _owner: self.into(), + }) + } } =20 impl Deref for Object { @@ -223,3 +242,34 @@ impl driver::AllocImpl for Object { dumb_map_offset: None, }; } + +/// An owned reference to a scatter-gather table of DMA address spans for = a GEM shmem object. +/// +/// This object holds an owned reference to the underlying GEM shmem objec= t, ensuring that the +/// [`scatterlist::SGTable`] referenced by this type remains valid for the= lifetime of this object. +/// +/// # Invariants +/// +/// - `sgt` is kept alive by `_owner`, ensuring it remains valid for as lo= ng as `Self`. +/// - `sgt` corresponds to the owned object in `_owner`. +/// - This object is only exposed in situations where we know the underlyi= ng `SGTable` will not be +/// modified for the lifetime of this object. Thus, it is safe to send/a= ccess this type across +/// threads. +pub struct SGTable { + sgt: NonNull, + _owner: ARef>, +} + +// SAFETY: This object is thread-safe via our type invariants. +unsafe impl Send for SGTable {} +// SAFETY: This object is thread-safe via our type invariants. +unsafe impl Sync for SGTable {} + +impl Deref for SGTable { + type Target =3D scatterlist::SGTable; + + fn deref(&self) -> &Self::Target { + // SAFETY: Creating an immutable reference to this is safe via our= type invariants. + unsafe { self.sgt.as_ref() } + } +} --=20 2.51.0