Re: [PATCH v3 2/4] media: mali-c55: Implement CCM block validation

[Laurent Pinchart]

On Mon, Jun 29, 2026 at 12:08:08PM +0100, Vincenzo Frascino wrote:
> On 29/06/2026 10:57, Laurent Pinchart wrote:
> > On Sat, Jun 27, 2026 at 04:29:14PM +0200, Jacopo Mondi wrote:
> >> From: Jacopo Mondi <jacopo.mondi+renesas@xxxxxxxxxxxxxxxx>
> >>
> >> Implement validation of CCM block parameters.
> >>
> >> CCM coefficients are expressed as 13 bits signed Q4.8 format and their
> >> raw value cannot be higher than 8191 (BIT(13) - 1).
> >>
> >> CCM gains are expressed as unsigned 12 bits Q4.8 format and their raw
> >> value cannot be higher than 4095 (BIT(12) - 1).
> >>
> >> CCM offsets are 12 bits unsigned integers and their value cannot be
> >> higher than 4095 (BIT(12) - 1).
> >>
> >> Validate the parameters provided by userspace using the .block_validate
> >> callback of struct v4l2_isp_params_block_type_info.
> > I don't think this is needed.
> > 
> > We need to validate parameters that can cause the ISP to malfunction in
> > ways that requires a system reset, or in ways that cause malfunction of
> > other system components (e.g. buffer overflows, memory bus lock ups,
> > ...). The rest doesn't need to be validated.
> > 
> > If you want to be cautious, you can just mask the value when writing to
> > registers, which I think you're doing in patch 1/4.
> 
> According to me here is not a matter of being cautious, but of honouring the
> contract with the userspace.
> 
> If the userspace is doing something wrong it should be notified. The only
> reasonable argument against this would be if this code is on a critical path and
> the validations have a performance impact.

I don't agree with this. As long as it doesn't have an impact on other
parts of the system, there's no need to notify userspace. It's purely a
userspace issue, it's pointless to waste CPU cycles every frame.

> @Jacopo, can you please confirm if this is the case?

-- 
Regards,

Laurent Pinchart