Re: [PATCH v2] drm/msm/dsi: use RMW cycles in dsi_update_dsc_timing

[Vinod Koul <vkoul@kernel.org>]

On 01-05-22, 22:41, Marijn Suijten wrote:
> On 2022-04-30 22:28:42, Dmitry Baryshkov wrote:
> > On 30/04/2022 21:58, Marijn Suijten wrote:
> > > On 2022-04-30 20:55:33, Dmitry Baryshkov wrote:
> > >> The downstream uses read-modify-write for updating command mode
> > >> compression registers. Let's follow this approach. This also fixes the
> > >> following warning:
> > >>
> > >> drivers/gpu/drm/msm/dsi/dsi_host.c:918:23: warning: variable 'reg_ctrl' set but not used [-Wunused-but-set-variable]
> > >>
> > >> Reported-by: kernel test robot <lkp@intel.com>
> > >> Fixes: 08802f515c3c ("drm/msm/dsi: Add support for DSC configuration")
> > >> Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> > > 
> > > I pointed this out in review multiple times, so you'll obviously get my:
> > 
> > I think I might have also pointed this out once (and then forgot to 
> > check that the issue was fixed by Vinod).
> > 
> > > 
> > > Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
> > > 
> > > (But are you sure there's nothing else to clear in the 1st CTRL
> > > register, only the lowest 16 bits?  That should mean `reg` never
> > > contains anything in 0xffff0000)
> > 
> > Judging from the downstream the upper half conains the same fields, but 
> > used for other virtual channel. I didn't research what's the difference 
> > yet. All the dtsi files that I have here at hand use 
> > 'qcom,mdss-dsi-virtual-channel-id = <0>;'
> 
> As replied to Abhinav I'm simply asking whether we should be strict
> and add `reg & 0xffff` to prevent accidentally writing the top 16 bits,
> which are stream 1.  It doesn't seem like the current code can hit that
> though, with all the macros using masks internally already; but it's

Since the macros were used I skipped setting that up explictly...

> still a little scary since we're assuming the registers for VIDEO are
> identical to CMD (as mentioned in the reply to Abhinav: I wonder if it's

The documentation seems to indicate they are similar and that is the
reason, I merged the code paths and set different registers required for
video and cmd modes

-- 
~Vinod