Re: [PATCH RESEND 2/2] pwm: mtk-disp: Configure double buffering before reading in .get_state()

[Alexandre Mergnat]

On 03/04/2023 15:30, 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@xxxxxxxxxxxxx>
> Reviewed-by: Nícolas F. R. A. Prado <nfraprado@xxxxxxxxxxxxx>
> Tested-by: Nícolas F. R. A. Prado <nfraprado@xxxxxxxxxxxxx>
> ---
>   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)
> +		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);

Hi,

Same test.
Looks fine.

Reviewed-by: Alexandre Mergnat <amergnat@xxxxxxxxxxxx>
Tested-by: Alexandre Mergnat <amergnat@xxxxxxxxxxxx>

Regards,
Alex