linux-kernel - Re: [PATCH v3 79/79] media: hantro: document the usage of pm_runtime_get

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Message-ID: <02948673-9572-a570-d28e-03a7208f39e1@arm.com>
Date:   Tue, 27 Apr 2021 16:08:47 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
Cc:     linuxarm@...wei.com, mauro.chehab@...wei.com,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH v3 79/79] media: hantro: document the usage of
 pm_runtime_get_sync()

On 2021-04-27 11:27, Mauro Carvalho Chehab wrote:
> Despite other *_get()/*_put() functions, where usage count is
> incremented only if not errors, the pm_runtime_get_sync() has
> a different behavior, incrementing the counter *even* on
> errors.
> 
> That's an error prone behavior, as people often forget to
> decrement the usage counter.
> 
> However, the hantro driver depends on this behavior, as it
> will decrement the usage_count unconditionally at the m2m
> job finish time, which makes sense.
> 
> So, intead of using the pm_runtime_resume_and_get() that
> would decrement the counter on error, keep the current
> API, but add a documentation explaining the rationale for
> keep using pm_runtime_get_sync().
> 
> Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@...nel.org>
> ---
>   drivers/staging/media/hantro/hantro_drv.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/staging/media/hantro/hantro_drv.c b/drivers/staging/media/hantro/hantro_drv.c
> index 595e82a82728..96f940c1c85c 100644
> --- a/drivers/staging/media/hantro/hantro_drv.c
> +++ b/drivers/staging/media/hantro/hantro_drv.c
> @@ -155,6 +155,13 @@ static void device_run(void *priv)
>   	ret = clk_bulk_enable(ctx->dev->variant->num_clocks, ctx->dev->clocks);
>   	if (ret)
>   		goto err_cancel_job;

..except this can also cause the same pm_runtime_put_autosuspend() call 
without even reaching the "matching" get below, so rather than some kind 
of cleverness it seems more like it's just broken :/

Robin.

> +
> +	/*
> +	 * The pm_runtime_get_sync() will increment dev->power.usage_count,
> +	 * even on errors. That's the expected behavior here, since the
> +	 * hantro_job_finish() function at the error handling code
> +	 * will internally call pm_runtime_put_autosuspend().
> +	 */
>   	ret = pm_runtime_get_sync(ctx->dev->dev);
>   	if (ret < 0)
>   		goto err_cancel_job;
>