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Message-ID: <8b9769f3-8a7c-3607-ca9a-09443cfbc9d9@collabora.com>
Date:   Thu, 27 Jul 2023 15:06:03 +0200
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Subject: Re: [PATCH RESEND v6 09/11] drm/mediatek: gamma: Add support for
 12-bit LUT and MT8195
Content-Language: en-US
To:     Alexandre Mergnat <amergnat@baylibre.com>, chunkuang.hu@kernel.org
Cc:     p.zabel@pengutronix.de, airlied@gmail.com, daniel@ffwll.ch,
        matthias.bgg@gmail.com, dri-devel@lists.freedesktop.org,
        linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, wenst@chromium.org,
        kernel@collabora.com, ehristev@collabora.com,
        "Jason-JH . Lin" <jason-jh.lin@mediatek.com>
References: <20230727094633.22505-1-angelogioacchino.delregno@collabora.com>
 <20230727094633.22505-10-angelogioacchino.delregno@collabora.com>
 <ec66e067-642e-1512-3e4b-b51065ccc75d@baylibre.com>
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
In-Reply-To: <ec66e067-642e-1512-3e4b-b51065ccc75d@baylibre.com>
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Precedence: bulk

Il 27/07/23 13:03, Alexandre Mergnat ha scritto:
> Hi Angelo !
> 
> On 27/07/2023 11:46, AngeloGioacchino Del Regno wrote:
>> Add support for 12-bit gamma lookup tables and introduce the first
>> user for it: MT8195.
>> While at it, also reorder the variables in mtk_gamma_set_common()
>> and rename `lut_base` to `lut0_base` to improve readability.
>>
>> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
>> Reviewed-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
>> ---
>>   drivers/gpu/drm/mediatek/mtk_disp_gamma.c | 61 ++++++++++++++++++-----
>>   1 file changed, 48 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/mediatek/mtk_disp_gamma.c 
>> b/drivers/gpu/drm/mediatek/mtk_disp_gamma.c
>> index f1a0b18b6c1a..e0e2d2bdbf59 100644
>> --- a/drivers/gpu/drm/mediatek/mtk_disp_gamma.c
>> +++ b/drivers/gpu/drm/mediatek/mtk_disp_gamma.c
>> @@ -27,12 +27,20 @@
>>   #define DISP_GAMMA_SIZE_VSIZE                GENMASK(12, 0)
>>   #define DISP_GAMMA_BANK                0x0100
>>   #define DISP_GAMMA_BANK_BANK                GENMASK(1, 0)
>> +#define DISP_GAMMA_BANK_DATA_MODE            BIT(2)
>>   #define DISP_GAMMA_LUT                0x0700
>> +#define DISP_GAMMA_LUT1                0x0b00
> 
> Is this offset generic to all MTK SoC which support this driver ?
> 
>> +/* For 10 bit LUT layout, R/G/B are in the same register */
>>   #define DISP_GAMMA_LUT_10BIT_R            GENMASK(29, 20)
>>   #define DISP_GAMMA_LUT_10BIT_G            GENMASK(19, 10)
>>   #define DISP_GAMMA_LUT_10BIT_B            GENMASK(9, 0)
>> +/* For 12 bit LUT layout, R/G are in LUT, B is in LUT1 */
> 
> As I understood from the application processor registers (v0.4), R/G are in LUT, B 
> is in LUT1 for 10bit and 12bit for MT8195. Can you check please to be sure ?
> 

That's right, but here I'm implying that 10-bit LUT is only for older SoCs, and
all of them have got the same register layout with one LUT register for R, G, B,
while all the new SoCs, which have got 12-bits LUT support, have got the new
register layout with two LUT registers (and multiple banks).
Infact, the MT8195 SoC was added here with 12-bits LUT support only (as the LUT
parameters extraction is easily handled by the drm_color_lut_extract() function).

The alternative would've been to add two compatibles, like
"mediatek,mt8195-disp-gamma-10bits" and "mediatek,mt8195-disp-gamma-12bits",
or a boolean property like "mediatek,lut-12bits" which would appear literally
everywhere starting from a certain point in time (since there's no reason to
use 10-bits LUT on MT8195, that starts now!).

Even then, consider the complication in code, where mtk_gamma_set_common()
would have to handle:
- 10-bits, layout A
- 10-bits, layout B -> but fallback to layout A if this is AAL
- 12-bits layout

is_aal = !(gamma && gamma->data);

for_each_bank()
{
	if (num_lut_banks > 1) write_num_bank();

	for (i = 0; i < lut_bank_size; i++) {
		.......

		if (!lut_diff || (i % 2 == 0)) {
			if (lut_bits == 12 || (lut_bits == 10 && layout_b)) {
				... setup word[0],[1] ...
			} else if (layout_b && !is_aal) {
				...setup word[0],[1]...
			} else {
				...setup word[0]
			}
		} else {
			 ^^^ almost repeat the same ^^^
		}
		writel(word[0], (...));
		if (lut_bits == 12 || (lut_bits == 10 && layout_b) && !is_aal)
			writel(word[i] (....));
	}
}

probe() {
	if (of_property_read_bool(dev->of_node, "mediatek,lut-12bits") ||
	    data->supports_only_12bits)
		priv->lut_bits = 12;
	else
		priv->lut_bits = 10;
}

...at least, that's the implementation that I would do to solve your concern,
which isn't *too bad*, but still, a big question arises here...


Why should we care about supporting *both* 10-bit and 12-bit Gamma LUTs on
the *same* SoC?


A 12-bit LUT gives us more precision and there's no penalty if we want to
convert a 10-bit LUT to a 12-bits one, as we're simply "ignoring" the value
of two bits per component (no expensive calculation involved)...

Is there anything that I'm underestimating here?

Cheers,
Angelo

>> +#define DISP_GAMMA_LUT_12BIT_R            GENMASK(11, 0)
>> +#define DISP_GAMMA_LUT_12BIT_G            GENMASK(23, 12)
>> +#define DISP_GAMMA_LUT_12BIT_B            GENMASK(11, 0)
>> +
>>   #define LUT_10BIT_MASK                0x03ff
>>   #define LUT_BITS_DEFAULT            10
>>   #define LUT_SIZE_DEFAULT            512
>> @@ -83,14 +91,15 @@ unsigned int mtk_gamma_get_lut_size(struct device *dev)
>>   void mtk_gamma_set_common(struct device *dev, void __iomem *regs, struct 
>> drm_crtc_state *state)
>>   {
>>       struct mtk_disp_gamma *gamma = dev_get_drvdata(dev);
>> -    unsigned int i;
>> +    void __iomem *lut0_base = regs + DISP_GAMMA_LUT;
>> +    void __iomem *lut1_base = regs + DISP_GAMMA_LUT1;
>> +    u32 cfg_val, data_mode, lbank_val, word[2];
>> +    int cur_bank, num_lut_banks;
>> +    u16 lut_bank_size, lut_size;
>>       struct drm_color_lut *lut;
>> -    void __iomem *lut_base;
>> +    unsigned int i;
>>       bool lut_diff;
>> -    u16 lut_bank_size, lut_size;
>>       u8 lut_bits;
>> -    u32 cfg_val, lbank_val, word;
>> -    int cur_bank, num_lut_banks;
>>       /* If there's no gamma lut there's nothing to do here. */
>>       if (!state->gamma_lut)
>> @@ -110,14 +119,17 @@ void mtk_gamma_set_common(struct device *dev, void __iomem 
>> *regs, struct drm_crt
>>       num_lut_banks = lut_size / lut_bank_size;
>>       cfg_val = readl(regs + DISP_GAMMA_CFG);
>> -    lut_base = regs + DISP_GAMMA_LUT;
>>       lut = (struct drm_color_lut *)state->gamma_lut->data;
>> +    /* Switch to 12 bits data mode if supported */
>> +    data_mode = FIELD_PREP(DISP_GAMMA_BANK_DATA_MODE, !!(lut_bits == 12));
>> +
>>       for (cur_bank = 0; cur_bank < num_lut_banks; cur_bank++) {
>>           /* Switch gamma bank and set data mode before writing LUT */
>>           if (num_lut_banks > 1) {
>>               lbank_val = FIELD_PREP(DISP_GAMMA_BANK_BANK, cur_bank);
>> +            lbank_val |= data_mode;
>>               writel(lbank_val, regs + DISP_GAMMA_BANK);
>>           }
>> @@ -130,9 +142,15 @@ void mtk_gamma_set_common(struct device *dev, void __iomem 
>> *regs, struct drm_crt
>>               hwlut.blue = drm_color_lut_extract(lut[n].blue, lut_bits);
>>               if (!lut_diff || (i % 2 == 0)) {
>> -                word = FIELD_PREP(DISP_GAMMA_LUT_10BIT_R, hwlut.red);
>> -                word |= FIELD_PREP(DISP_GAMMA_LUT_10BIT_G, hwlut.green);
>> -                word |= FIELD_PREP(DISP_GAMMA_LUT_10BIT_B, hwlut.blue);
>> +                if (lut_bits == 12) {
>> +                    word[0] = FIELD_PREP(DISP_GAMMA_LUT_12BIT_R, hwlut.red);
>> +                    word[0] |= FIELD_PREP(DISP_GAMMA_LUT_12BIT_G, hwlut.green);
>> +                    word[1] = FIELD_PREP(DISP_GAMMA_LUT_12BIT_B, hwlut.blue);
>> +                } else {
>> +                    word[0] = FIELD_PREP(DISP_GAMMA_LUT_10BIT_R, hwlut.red);
>> +                    word[0] |= FIELD_PREP(DISP_GAMMA_LUT_10BIT_G, hwlut.green);
>> +                    word[0] |= FIELD_PREP(DISP_GAMMA_LUT_10BIT_B, hwlut.blue);
>> +                }
>>               } else {
>>                   diff.red = lut[n].red - lut[n - 1].red;
>>                   diff.red = drm_color_lut_extract(diff.red, lut_bits);
>> @@ -143,11 +161,19 @@ void mtk_gamma_set_common(struct device *dev, void __iomem 
>> *regs, struct drm_crt
>>                   diff.blue = lut[n].blue - lut[n - 1].blue;
>>                   diff.blue = drm_color_lut_extract(diff.blue, lut_bits);
>> -                word = FIELD_PREP(DISP_GAMMA_LUT_10BIT_R, diff.red);
>> -                word |= FIELD_PREP(DISP_GAMMA_LUT_10BIT_G, diff.green);
>> -                word |= FIELD_PREP(DISP_GAMMA_LUT_10BIT_B, diff.blue);
>> +                if (lut_bits == 12) {
>> +                    word[0] = FIELD_PREP(DISP_GAMMA_LUT_12BIT_R, diff.red);
>> +                    word[0] |= FIELD_PREP(DISP_GAMMA_LUT_12BIT_G, diff.green);
>> +                    word[1] = FIELD_PREP(DISP_GAMMA_LUT_12BIT_B, diff.blue);
>> +                } else {
>> +                    word[0] = FIELD_PREP(DISP_GAMMA_LUT_10BIT_R, diff.red);
>> +                    word[0] |= FIELD_PREP(DISP_GAMMA_LUT_10BIT_G, diff.green);
>> +                    word[0] |= FIELD_PREP(DISP_GAMMA_LUT_10BIT_B, diff.blue);
>> +                }
>>               }
>> -            writel(word, (lut_base + i * 4));
>> +            writel(word[0], (lut0_base + i * 4));
>> +            if (lut_bits == 12)
>> +                writel(word[1], (lut1_base + i * 4));
> 
> ditto
> 
>>           }
>>       }
>> @@ -271,11 +297,20 @@ static const struct mtk_disp_gamma_data 
>> mt8183_gamma_driver_data = {
>>       .lut_size = 512,
>>   };
>> +static const struct mtk_disp_gamma_data mt8195_gamma_driver_data = {
>> +    .lut_bank_size = 256,
>> +    .lut_bits = 12,
> 
> If I'm right, ".lut_bits = 10" will not work properly.
> 
>> +    .lut_diff = true,
>> +    .lut_size = 1024,
>> +};
>> +
>>   static const struct of_device_id mtk_disp_gamma_driver_dt_match[] = {
>>       { .compatible = "mediatek,mt8173-disp-gamma",
>>         .data = &mt8173_gamma_driver_data},
>>       { .compatible = "mediatek,mt8183-disp-gamma",
>>         .data = &mt8183_gamma_driver_data},
>> +    { .compatible = "mediatek,mt8195-disp-gamma",
>> +      .data = &mt8195_gamma_driver_data},
>>       {},
>>   };
>>   MODULE_DEVICE_TABLE(of, mtk_disp_gamma_driver_dt_match);
>