Re: [PATCH] drm/etnaviv: fix perfmon domain interation

[Lucas Stach <l.stach@pengutronix.de>]

Hi Christian,

Am Montag, den 11.05.2020, 14:37 +0200 schrieb Christian Gmeiner:
> The GC860 has one GPU device which has a 2d and 3d core. In this case
> we want to expose perfmon information for both cores.
> 
> The driver has one array which contains all possible perfmon domains
> with some meta data - doms_meta. Here we can see that for the GC860
> two elements of that array are relevant:
> 
>   doms_3d: is at index 0 in the doms_meta array with 8 perfmon domains
>   doms_2d: is at index 1 in the doms_meta array with 1 perfmon domain
> 
> The userspace driver wants to get a list of all perfmon domains and
> their perfmon signals. This is done by iterating over all domains and
> their signals. If the userspace driver wants to access the domain with
> id 8 the kernel driver fails and returns invalid data from doms_3d with
> and invalid offset.
> 
> This results in:
>   Unable to handle kernel paging request at virtual address 00000000
> 
> On such a device it is not possible to use the userspace driver at all.
> 
> The fix for this off-by-one error is quite simple.
> 
> Reported-by: Paul Cercueil <paul@crapouillou.net>
> Tested-by: Paul Cercueil <paul@crapouillou.net>
> Fixes: ed1dd899baa3 ("drm/etnaviv: rework perfmon query infrastructure")
> Cc: stable@vger.kernel.or

Missing last letter of the TLD.

> Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
> ---
>  drivers/gpu/drm/etnaviv/etnaviv_perfmon.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_perfmon.c b/drivers/gpu/drm/etnaviv/etnaviv_perfmon.c
> index e6795bafcbb9..35f7171e779a 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_perfmon.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_perfmon.c
> @@ -453,7 +453,7 @@ static const struct etnaviv_pm_domain *pm_domain(const struct etnaviv_gpu *gpu,
>  		if (!(gpu->identity.features & meta->feature))
>  			continue;
>  
> -		if (meta->nr_domains < (index - offset)) {
> +		if ((meta->nr_domains - 1) < (index - offset)) {

While the logic is correct, I find this quite hard to read. A more
idiomatic way to write this (which is much easier to grok when reading
the code IMHO) would be:

if (index - offset >= meta->nr_domains)

If you agree, please send a v2 of this patch.

Regards,
Lucas
>  			offset += meta->nr_domains;
>  			continue;
>  		}