Re: [PATCH] drm: mxsfb: Clear FIFO_CLEAR bit

[Marek Vasut <marex@denx.de>]

On 6/28/21 10:09 AM, Lucas Stach wrote:
> Am Samstag, dem 26.06.2021 um 20:15 +0200 schrieb Marek Vasut:
>> On 6/24/21 2:01 PM, Lucas Stach wrote:
>>> Am Dienstag, dem 22.06.2021 um 11:33 +0200 schrieb Marek Vasut:
>>>> On 6/22/21 9:28 AM, Lucas Stach wrote:
>>>>> Am Montag, dem 21.06.2021 um 18:30 +0200 schrieb Marek Vasut:
>>>>>> On 6/21/21 2:14 PM, Lucas Stach wrote:
>>>>>>
>>>>>> [...]
>>>>>>
>>>>>>>> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_kms.c b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
>>>>>>>> index 98d8ba0bae84..22cb749fc9bc 100644
>>>>>>>> --- a/drivers/gpu/drm/mxsfb/mxsfb_kms.c
>>>>>>>> +++ b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
>>>>>>>> @@ -241,6 +241,9 @@ static void mxsfb_crtc_mode_set_nofb(struct mxsfb_drm_private *mxsfb,
>>>>>>>>      
>>>>>>>>      	/* Clear the FIFOs */
>>>>>>>>      	writel(CTRL1_FIFO_CLEAR, mxsfb->base + LCDC_CTRL1 + REG_SET);
>>>>>>>> +	readl(mxsfb->base + LCDC_CTRL1);
>>>>>>>
>>>>>>> Do you really need those readbacks? As both writes are targeting the
>>>>>>> same slave interface, the memory barrier in the clear write should push
>>>>>>> the set write.
>>>>>>
>>>>>> What would push the clear write then ? We can drop one of the readl()s,
>>>>>> but not the last one.
>>>>>
>>>>> There are a lot of more writes with barriers to the controller slave
>>>>> interface in that function after clearing the FIFO. I don't see why
>>>>> this readback would be required.
>>>>
>>>> Because you really do want to make sure the fifo is cleared before you
>>>> start doing any of those other writes or configuring the controller in
>>>> any way.
>>>
>>> I still don't see the reason. What additional properties do you think
>>> the readback provides that isn't already provided by the barriers in
>>> the following writes?
>>
>> See the paragraph above -- we have to make sure the writes that trigger
>> the FIFO clearing really take place before any other writes do.
> 
> And they do, as there are write barriers prepended to the writes
> following the FIFO clear. The readback just lets the CPU wait until the
> write reached the peripheral, which I don't see a reason to do here.
> The ordering of the writes from the perspective of the peripheral is
> completely the same with or without the readback. The later writes can
> not overtake the FIFO clear writes due to the barriers.
> 
> I'm strongly against adding stuff because it "might have an effect", if
> it isn't required by architectural rules. It clutters the code and some
> months/years down the line nobody dares to cleanup/remove this stuff
> anymore, because everyone assumes that there was a good reason for
> adding those things.

Since there is no RTL for any of the iMXes or their IPs, how do you 
propose anyone except NXP can validate what is and what is not required ?

This patch helps with a problem where I sporadically observe shifted 
image on boot on mx8mm.