linux-kernel - Re: [PATCH 1/3] drm/panthor: Update memattr programing to align with GPU spec

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Message-ID: <20241025143144.625df2b7@collabora.com>
Date: Fri, 25 Oct 2024 14:31:44 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Liviu Dudau <liviu.dudau@....com>
Cc: Akash Goel <akash.goel@....com>, Robin Murphy <robin.murphy@....com>,
 steven.price@....com, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, mihail.atanassov@....com,
 ketil.johnsen@....com, florent.tomasin@....com,
 maarten.lankhorst@...ux.intel.com, mripard@...nel.org, tzimmermann@...e.de,
 airlied@...il.com, daniel@...ll.ch, nd@....com
Subject: Re: [PATCH 1/3] drm/panthor: Update memattr programing to align
 with GPU spec

On Fri, 25 Oct 2024 10:24:32 +0100
Liviu Dudau <liviu.dudau@....com> wrote:

> On Thu, Oct 24, 2024 at 05:49:44PM +0200, Boris Brezillon wrote:
> > +Robin for the MMU details
> > 
> > On Thu, 24 Oct 2024 15:54:30 +0100
> > Akash Goel <akash.goel@....com> wrote:
> >   
> > > Mali GPU Arch spec forbids the GPU PTEs to indicate Inner or Outer
> > > shareability when no_coherency protocol is selected. Doing so results in
> > > unexpected or undesired snooping of the CPU caches on some platforms,
> > > such as Juno FPGA, causing functional issues. For example the boot of
> > > MCU firmware fails as GPU ends up reading stale data for the FW memory
> > > pages from the CPU's cache. The FW memory pages are initialized with
> > > uncached mapping when the device is not reported to be dma-coherent.
> > > The shareability bits are set to inner-shareable when IOMMU_CACHE flag
> > > is passed to map_pages() callback and IOMMU_CACHE flag is passed by
> > > Panthor driver when memory needs to be mapped as cached on the GPU side.
> > > 
> > > IOMMU_CACHE seems to imply cache coherent and is probably not fit for
> > > purpose for the memory that is mapped as cached on GPU side but doesn't
> > > need to remain coherent with the CPU.  
> > 
> > Yeah, IIRC I've been abusing the _CACHE flag to mean GPU-cached, not
> > cache-coherent. I think it be good to sit down with Rob and add the
> > necessary IOMMU_ flags so we can express all the shareability and
> > cacheability variants we have with the "Mali" MMU. For instance, I
> > think the shareability between MCU/GPU can be expressed properly at the
> > moment, and we unconditionally map things uncached because of that.  
> 
> Boris, did you mean to say "shareability between MCU/GPU *can't* be expressed
> properly" ? Currently the sentence reads a bit strange, as if there was a
> negation somewhere.

Yes, sorry, I meant "can't".