linux-kernel - Re: [PATCH v2 4/4] drm/panfrost: Handle non-aligned lock addresses

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Message-ID: <YSZOgzaYN2Vub4w+@maud>
Date:   Wed, 25 Aug 2021 10:07:21 -0400
From:   Alyssa Rosenzweig <alyssa@...labora.com>
To:     Steven Price <steven.price@....com>
Cc:     Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        dri-devel@...ts.freedesktop.org, Rob Herring <robh@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] drm/panfrost: Handle non-aligned lock addresses

> > In practice, the current callers pass PAGE_SIZE aligned inputs, avoiding
> > the bug. Therefore this doesn't need to be backported. Still, that's a
> > happy accident and not a precondition of lock_region, so we let's do the
> > right thing to future proof.
> 
> Actually it's worse than that due to the hardware behaviour, the spec
> states (for LOCKADDR_BASE):
> 
> > Only the upper bits of the address are used. The address is aligned to a
> > multiple of the region size, so a variable number of low-order bits are
> > ignored, depending on the selected region size. It is recommended that software
> > ensures that these low bits in the address are cleared, to avoid confusion.
> 
> It appears that indeed this has caused confusion ;)
> 
> So for a simple request like locking from 0xCAFE0000 - 0xCB010000 (size
> = 0x30000) the region width gets rounded up (to 0x40000) which causes
> the start address to be effectively rounded down (by the hardware) to
> 0xCAFC0000 and we fail to lock 0xCB000000-0xCB010000.
> 
> Interestingly (unless my reading of this is wrong) that means to lock
> 0xFFFF0000-0x100010000 (i.e. crossing the 4GB boundary) requires locking
> *at least* 0x00000000-0x200000000 (i.e. locking the first 8GB).
> 
> This appears to be broken in kbase (which actually does zero out the low
> bits of the address) - I've raised a bug internally so hopefully someone
> will tell me if I've read the spec completely wrong here.

Horrifying, and not what I wanted to read my last day before 2 weeks of
leave. Let's drop this patch, hopefully by the time I'm back, your
friends in GPU can confirm that's a spec bug and not an actual
hardware/driver one...

Can you apply the other 3 patches in the mean time? Thanks :-)