[PATCH] drm: lcdif: change burst size to 256B

[Marek Vasut]

On 7/27/22 05:56, Marco Felsch wrote:
> Hi Marek, Liu,

Hi,

> On 22-07-26, Liu Ying wrote:
>> On Tue, 2022-07-26 at 16:19 +0200, Marek Vasut wrote:
>>> On 7/26/22 11:43, Marco Felsch wrote:
>>>> FIFO underruns are seen if a AXI bus master with a higher priority
>>>> do a
>>>> lot of memory access. Increase the burst size to 256B to avoid such
>>>> underruns and to improve the memory access efficiency.
>>>
>>> Sigh, this again ...
> 
> I know.. we also tried the PANIC mode but this somehow didn't worked as
> documented. So this was the only way to reduce the underruns without
> adapting the interconnect prio for the hdmi-lcdif.

Right, the PANIC watermark didn't work on mxsfb for me either when it 
came to FIFO underruns.

[...]

>>>> +	 * are two known values:
>>>> +	 *  1 - 128Byte
>>>> +	 *  2 - 256Byte
>>>> +	 *
>>>> +	 * Downstream has set the burst size to 256Byte to improve the
>>>> memory
>>>> +	 * efficiency so set it here too. This also reduces the FIFO
>>>> underrun
>>>> +	 * possibility.
>>>> +	 */
>>>> +	ctrl = CTRLDESCL0_3_P_SIZE(2) | CTRLDESCL0_3_T_SIZE(2) |
>>>> +	       CTRLDESCL0_3_PITCH(lcdif->crtc.primary->state->fb-
>>>>> pitches[0]);
>>>> +	writel(ctrl, lcdif->base + LCDC_V8_CTRLDESCL0_3);
>>
>> Nit: I would write the register directly, instead of caching the value
>> in ctrl.
> 
> IMHO it's more readable that way.

I agree, and we can also add to the variable in case there are more 
undocumented bits.