Re: [PATCH] drm/panthor: Ensure MCU is disabled on suspend

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 9 Oct 2025 09:42:08 +0100
Karunika Choo <karunika.choo@arm.com> wrote:

> On 08/10/2025 12:32, Boris Brezillon wrote:
> > On Wed,  8 Oct 2025 12:51:11 +0200
> > Ketil Johnsen <ketil.johnsen@arm.com> wrote:
> >   
> >> Currently the Panthor driver needs the GPU to be powered down
> >> between suspend and resume. If this is not done, then the
> >> MCU_CONTROL register will be preserved as AUTO, which again will
> >> cause a premature FW boot on resume. The FW will go directly into
> >> fatal state in this case.
> >>
> >> This case needs to be handled as there is no guarantee that the
> >> GPU will be powered down after the suspend callback on all platforms.
> >>
> >> The fix is to call panthor_fw_stop() in "pre-reset" path to ensure
> >> the MCU_CONTROL register is cleared (set DISABLE). This matches
> >> well with the already existing call to panthor_fw_start() from the
> >> "post-reset" path.
> >>
> >> Signed-off-by: Ketil Johnsen <ketil.johnsen@arm.com>
> >> ---
> >>  drivers/gpu/drm/panthor/panthor_fw.c | 1 +
> >>  1 file changed, 1 insertion(+)
> >>
> >> diff --git a/drivers/gpu/drm/panthor/panthor_fw.c b/drivers/gpu/drm/panthor/panthor_fw.c
> >> index 9bf06e55eaee..df767e82148a 100644
> >> --- a/drivers/gpu/drm/panthor/panthor_fw.c
> >> +++ b/drivers/gpu/drm/panthor/panthor_fw.c
> >> @@ -1099,6 +1099,7 @@ void panthor_fw_pre_reset(struct panthor_device *ptdev, bool on_hang)
> >>  	}
> >>  
> >>  	panthor_job_irq_suspend(&ptdev->fw->irq);
> >> +	panthor_fw_stop(ptdev);  
> > 
> > Is this not preventing fast resets? My understanding was that
> > MCU_CONTROL shouldn't be touched if the MCU was halted from the FW, but
> > maybe I got that wrong. If it's just for the MCU crash case, we can
> > have:
> > 
> > 	if (on_hang)
> > 		panthor_fw_stop(ptdev);
> >   
> 
> Hi Boris, I think as long as the FW is properly halted, we can safely
> disable the MCU. In fact, because of the ptdev->reset.fast tracking, we
> can call panthor_fw_stop() in both cases, as the flag allows us to
> handle the resume path correctly.
> 
> As per Ketil's commit message, if we don't clear the HALT bit on L2
> power on, the MCU can start booting the FW with the bit enabled, which
> in certain cases can make it fail to boot. So this patch fixes that by
> gating when the FW is allowed to boot.
> 
> I also believe this behaviour will be better aligned with the expected
> behaviour from the FW of newer GPUs (Mali-G1 for example).

Okay, as long as you're sure it doesn't screw up the fast reset, I'm
happy to get that in.

Acked-by: Boris Brezillon <boris.brezillon@collabora.com>