Re: [PATCH v5 2/7] drm/ingenic: Add support for JZ4780 and HDMI output

["H. Nikolaus Schaller" <hns@goldelico.com>]

Hi Paul,

> Am 08.11.2021 um 17:30 schrieb Paul Cercueil <paul@crapouillou.net>:
> 
> Hi,
> 
> Le lun., nov. 8 2021 at 16:29:11 +0100, H. Nikolaus Schaller <hns@goldelico.com> a écrit :
>> Bnjour Paul,
>>> Am 08.11.2021 um 13:20 schrieb Paul Cercueil <paul@crapouillou.net>:
>>> Hi,
>>>> e.g. jz4770.dtsi:
>>>> 	lcd: lcd-controller@13050000 {
>>>> 		compatible = "ingenic,jz4770-lcd";
>>>> 		reg = <0x13050000 0x300>;
>>>> or jz4725b.dtsi:
>>>> 	lcd: lcd-controller@13050000 {
>>>> 		compatible = "ingenic,jz4725b-lcd";
>>>> 		reg = <0x13050000 0x1000>;
>>>> So max_register becomes 0x300 or 0x1000 but not
>>>> #define JZ_REG_LCD_SIZE1	0x12c
>>>> 	.max_reg = JZ_REG_LCD_SIZE1,
>>>> And therefore wastes a lot of regmap memory.
>>> "regmap memory"? ...
>> regmap allocates memory for its cache. Usually the total amount specified in the reg property.
> 
> We are not using any register cache here.
> 
>>>> Do you want this? DTS should not be reduced (DTS should be kept as stable as possible), since the reg property describes address mapping - not how many bytes are really used by registers or how big a cache should be allocated (cache allocation size requirements are not hardware description).
>>> The DTS should list the address and size of the register area. If your last register is at address 0x12c and there's nothing above, then the size in DTS should be 0x130.
>> If I look into some .dtsi it is sometimes that way but sometimes not. There seems to be no consistent rule.
>> So does this mean you allow me to modify jz4740.dtsi, jz4770.dtsi and jz4725b.dtsi as well (as mentioned above: this is beyond the scope of my project)?
> 
> You could update them if you wanted to, but there is no need to do it here.

Hm. Then we are changing the .max_register initialization to a much bigger value.

> 
>>>> But here are good news:
>>>> I have a simpler and less invasive proposal. We keep the devm_regmap_init_mmio code as is and just increase its .max_register from JZ_REG_LCD_SIZE1 to JZ_REG_LCD_PCFG when introducing the jz4780. This wastes a handful bytes for all non-jz4780 chips but less than using the DTS memory region size. And is less code (no entry in soc_info tables, no modifyable copy) and faster code execution than all other proposals.
>>>> This is then just a single-line change when introducing the jz4780. And no "preparation for adding jz4780" patch is needed at all. No patch to split out for separate review.
>>>> Let's go this way to get it eventually finalized. Ok?
>>> No.
>> Look friend, if you explain your "no" and what is wrong with my arguments, it helps to understand your decisions and learn something from them. A plain "no" does not help anyone.
> 
> I answered just "no" because I felt like I explained already what I wanted to see in the previous email.
> 
> By using a huge number as the .max_register, we do *not* waste additional memory. Computing the value of the .max_register field does not add any overhead, either.
> 
> The .max_register is only used for boundary checking. To make sure that you're not calling regmap_write() with an invalid register. That's all there is to it.

Ah, now I understand our disconnect. So far I have used regmaps mainly for i2c devices and there is caching to avoid redundant i2c traffic...

So I just assumed wrongly that the regmap driver also allocates some buffer/cache here. Although it does not initialize .cache_type (default: REGCACHE_NONE).

> 
>> So to summarize: if you prefer something which I consider worse, it is ok for me... In the end you are right - you are the maintainer, not me. So you have to live with your proposals.
>> Therefore, I have prepared new variants so you can choose which one is easier to maintain for you.
>> Note that they are both preparing for full jz4780-lcdc/hdmi support but in very different ways:
>> Variant 1 already adds some jz4780 stuff while Variant 2 just prepares for it.
>> Variant 2 is not tested (except to compile). So it needs some Tested-by: from someone with access to hardware. IMHO it is more invasive.
>> And don't forget: DTB could be in ROM or be provided by a separate bootloader... So we should not change it too often (I had such discussions some years ago with maintainers when I thought it is easier to change DTS instead of code).
>> Variant 3 would be to not separate this. As proposed in [PATCH v5 2/7].
>> (Finally, a Variant 3b would be to combine the simple change from Variant 1 with Variant 3).
>> So what is your choice?
> 
> Variant 4: the variant #2 without the changes to the DTSI files.

Hm. If there is no cache and we can safely remove tight boundary checking (by JZ_REG_LCD_SIZE1) for jz4725/40/70 (by not fixing DTSI) why do we still need the max_register calculation from DTSI specifically for jz4780 and at all?

So what about:

Variant 5: set .max_register = 0x1800, i.e. "big enough for everyone" (includes z4780 gamma and vee registers) + no DTSI changes (+ no jz4780 register constants like Variant 1)

+ no DTSI changes
+ no calculation from DTSI needed
+ single separate patch to prepare for jz4780 but not included in jz4780 patch

BR and thanks,
Nikolaus