Re: [PATCH] drm/bridge/synopsys: dsi: use common mipi_dsi_create_packet()

[Philippe CORNU]

Hi Brian,

On 01/23/2018 10:15 PM, Brian Norris wrote:
> Hi Philippe,
> 
> On Thu, Jan 18, 2018 at 11:40:48AM +0000, Philippe CORNU wrote:
>> On 01/11/2018 12:16 PM, Philippe CORNU wrote:
>>> To be honest, I do not really like the memcpy here too and I agree with
>>> you regarding the BE issue.
>>>
>>> My first "stm" driver (ie. before using this "freescale/rockchip"
>>> dw-mipi-dsi driver with the memcpy) used the "exact" same code as the
>>> Tegra dsi tegra_dsi_writesl() function with the 2 loops.
>>>
>>> https://elixir.free-electrons.com/linux/v4.14/source/drivers/gpu/drm/tegra/dsi.c#L1248
>>>
>>>
>>> IMHO, it is better than memcpy...
>>> I added these 3 "documentation" lines, maybe we may reuse them or
>>> something similar...
>>>
>>> /*
>>>   Â* Write 8-bit payload data into the 32-bit payload data register.
>>>   Â* ex: payload data "0x01, 0x02, 0x03, 0x04, 0x05, 0x06" will become
>>>   Â* "0x04030201 0x00000605" 32-bit writes
>>>   Â*/
>>>
>>> Not sure it helps to fix the BE issue but we may add a TODO stating that
>>> "this loop has not been tested on BE"...
>>>
>>> What is your opinion?
> 
> I'm sorry, I don't think I noticed your reply here. I'm trying to unbury
> some email, but that's sometimes a losing battle...
> 
> That code actually does look correct, and it's perhaps marginally
> better-looking in my opinion. It's up to you if you want to propose
> another patch :) At this point, it's only a matter of nice code, not
> correctness I believe.
> 
>> As your patch has been merged, I have few short questions and for each
>> related new patch, I would like to know if you prefer that I implement
>> it or if you prefer to do it by yourself, it's really like you want, on
>> my side, no problem to make them all, some or none, I don't want us to
>> implement these in parallel :-)
>>
>> * Do you have any opinion regarding Tegra-like loops vs the memcpy? (see
>> my comment above) If you think the Tegra-like loops is a better approach
>> than memcpy, there is a small patch to write.
> 
> My opinion is above.
> 

I do not know yet if I will send a patch but several reasons may push me 
to do it:
* Andrzej proposed a nicer code in his last review so it means the 
actual code with memcpy's is "not so nice" (even if it works fine)
* Several dsi drivers use the Tegra-like loops (Tegra, intel,... ) and 
in vc4/exynos/mtk drivers memcpy is not used, msm uses memcpy... well, 
not sure it is then a good argument, different solutions for different hw...
* Coming cadence dsi bridge driver uses Tegra-like loops.
* I think my read function will also have Tegra-like loops, if it is the 
case, it could be nice to have something homogeneous...

Anyway, it is not an important point : )

>> * Returned value with number of bytes received/transferred: there is a
>> small patch to write
> 
> I don't think I followed that one very well. I'm not sure my opinion
> really matters, as long as you get someone else to agree. I do not plan
> to write any such patch in the near term.
> 
>> * Regarding read operations: I propose to add a TODO + DRM_WARN in case
>> someone want to use the API for read operations. Note that I plan to
>> implement the read feature but I do not know yet when and maybe Rockchip
>> people already have something ~ready?
> 
> The warning would be nice to do now, regardless.
> 
> Rockchip folks wrote up something for read support here [1], but it's
> based on a semi-forked version of the driver (we're trying to clean up
> the divergence, but it's not there yet). Perhaps it would provide useful
> fodder for your work. I don't think Rockchip is immediately working on
> upstreaming this particular patch, so it's totally fair to handle it
> yourself. It's got the GPL sign-off ;)
> 
> Brian
> 
> [1] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/863347
> 

Very good information, I will have a look,
many thanks
Philippe :-)