Date:   Thu, 26 Jan 2023 10:19:14 -0500
From:   =?utf-8?B?TsOtY29sYXMgRi4gUi4gQS4=?= Prado 
        <nfraprado@collabora.com>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
Cc:     thierry.reding@gmail.com, u.kleine-koenig@pengutronix.de,
        matthias.bgg@gmail.com, weiqing.kong@mediatek.com,
        jitao.shi@mediatek.com, linux-pwm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org,
        kernel@collabora.com
Subject: Re: [PATCH 2/2] pwm: mtk-disp: Configure double buffering before
 reading in .get_state()
Message-ID: <20230126151914.rhwhioz6yyhaq3z2@notapiano>
References: <20230123160615.375969-1-angelogioacchino.delregno@collabora.com>
 <20230123160615.375969-3-angelogioacchino.delregno@collabora.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20230123160615.375969-3-angelogioacchino.delregno@collabora.com>
Precedence: bulk

On Mon, Jan 23, 2023 at 05:06:15PM +0100, AngeloGioacchino Del Regno wrote:
> The DISP_PWM controller's default behavior is to always use register
> double buffering: all reads/writes are then performed on shadow
> registers instead of working registers and this becomes an issue
> in case our chosen configuration in Linux is different from the
> default (or from the one that was pre-applied by the bootloader).
> 
> An example of broken behavior is when the controller is configured
> to use shadow registers, but this driver wants to configure it
> otherwise: what happens is that the .get_state() callback is called
> right after registering the pwmchip and checks whether the PWM is
> enabled by reading the DISP_PWM_EN register;
> At this point, if shadow registers are enabled but their content
> was not committed before booting Linux, we are *not* reading the
> current PWM enablement status, leading to the kernel knowing that
> the hardware is actually enabled when, in reality, it's not.
> 
> The aforementioned issue emerged since this driver was fixed with
> commit 0b5ef3429d8f ("pwm: mtk-disp: Fix the parameters calculated
> by the enabled flag of disp_pwm") making it to read the enablement
> status from the right register.
> 
> Configure the controller in the .get_state() callback to avoid
> this desync issue and get the backlight properly working again.
> 
> Fixes: 3f2b16734914 ("pwm: mtk-disp: Implement atomic API .get_state()")
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
> ---
>  drivers/pwm/pwm-mtk-disp.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/pwm/pwm-mtk-disp.c b/drivers/pwm/pwm-mtk-disp.c
> index 82b430d881a2..fe9593f968ee 100644
> --- a/drivers/pwm/pwm-mtk-disp.c
> +++ b/drivers/pwm/pwm-mtk-disp.c
> @@ -196,6 +196,16 @@ static int mtk_disp_pwm_get_state(struct pwm_chip *chip,
>  		return err;
>  	}
>  
> +	/*
> +	 * Apply DISP_PWM_DEBUG settings to choose whether to enable or disable
> +	 * registers double buffer and manual commit to working register before
> +	 * performing any read/write operation
> +	 */
> +	if (mdp->data->bls_debug)

I feel like this condition should be the same as in the apply() callback, since
they're doing the same write operation, so also have '&& !has_commit'.

Reviewed-by: N?colas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: N?colas F. R. A. Prado <nfraprado@collabora.com>

On MT8192 Asurada Spherion.

Thanks,
N?colas

> +		mtk_disp_pwm_update_bits(mdp, mdp->data->bls_debug,
> +					 mdp->data->bls_debug_mask,
> +					 mdp->data->bls_debug_mask);
> +
>  	rate = clk_get_rate(mdp->clk_main);
>  	con0 = readl(mdp->base + mdp->data->con0);
>  	con1 = readl(mdp->base + mdp->data->con1);
> -- 
> 2.39.0
> 
>