linux-kernel - Re: [PATCH 1/2] drm/nouveau/firmware: Fix SG_DEBUG error with nvkm_firmware

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Date: Sun, 28 Apr 2024 11:52:47 -0400
From: Lyude Paul <lyude@...hat.com>
To: Timur Tabi <ttabi@...dia.com>, "nouveau@...ts.freedesktop.org"
	 <nouveau@...ts.freedesktop.org>
Cc: "bskeggs@...hat.com" <bskeggs@...hat.com>, "kherbst@...hat.com"
 <kherbst@...hat.com>, "airlied@...hat.com" <airlied@...hat.com>, 
 "justinstitt@...gle.com" <justinstitt@...gle.com>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "keescook@...omium.org" <keescook@...omium.org>, "airlied@...il.com"
 <airlied@...il.com>,  "dakr@...hat.com" <dakr@...hat.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
 "daniel@...ll.ch" <daniel@...ll.ch>
Subject: Re: [PATCH 1/2] drm/nouveau/firmware: Fix SG_DEBUG error with
 nvkm_firmware_ctor()

On Fri, 2024-04-26 at 15:47 +0000, Timur Tabi wrote:
> On Fri, 2024-04-26 at 11:41 -0400, Lyude Paul wrote:
> > We hit this because when initializing firmware of type
> > NVKM_FIRMWARE_IMG_DMA we allocate coherent memory and then attempt
> > to
> > include that coherent memory in a scatterlist. 
> 
> I'm sure this patch is a good one, and I will try to test it soon,
> but I am
> very curious to know why including coherent memory in a scatterlist
> is bad.

Thanks for asking this as I think you unintentionally pointed out this
explanation I gave doesn't make sense - so I looked a bit more into it.
The issue isn't coherent memory in the scatterlist, the issue is that
we're allocating with dma_alloc_coherent(). And according to the source
in dma_alloc_attrs() (which dma_alloc_coherent() is just a wrapper)
for):

   /*
    * DMA allocations can never be turned back into a page pointer, so
    * requesting compound pages doesn't make sense (and can't even be
    * supported at all by various backends).
    */
   if (WARN_ON_ONCE(flag & __GFP_COMP))
   	return NULL;

Which explains the check in sg_set_buf() that this patch stops us from
hitting:

   BUG_ON(!virt_addr_valid(buf));

Scatterlists need page pointers (we use one later down here:)

   sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));

But we can't get a page pointer from an allocation made by
dma_alloc_coherent() - but we can from vmalloc(). I'll fix the patch
explanation in the next version, I have to send out another version
anyhow since I realized that patch #2 still needs one more check to
work properly
-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat