linux-kernel - Re: [PATCH v2 1/8] mmc: sdhci: Get rid of finish

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Date:   Mon, 25 Feb 2019 10:17:55 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Faiz Abbas <faiz_abbas@...com>, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-mmc@...r.kernel.org,
        linux-omap@...r.kernel.org
Cc:     ulf.hansson@...aro.org, robh+dt@...nel.org, mark.rutland@....com,
        kishon@...com, zhang.chunyan@...aro.org
Subject: Re: [PATCH v2 1/8] mmc: sdhci: Get rid of finish_tasklet

On 15/02/19 9:20 PM, Faiz Abbas wrote:
> sdhci.c has two bottom halves implemented. A threaded_irq for handling
> card insert/remove operations and a tasklet for finishing mmc requests.
> With the addition of external dma support, dmaengine APIs need to
> terminate in non-atomic context before unmapping the dma buffers.
> 
> To facilitate this, remove the finish_tasklet and move the call of
> sdhci_request_done() to the threaded_irq() callback.

The irq thread has a higher latency than the tasklet.  The performance drop
is measurable on the system I tried:

Before:

# dd if=/dev/mmcblk1 of=/dev/null bs=1G count=1 &
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 4.44502 s, 242 MB/s

After:

# dd if=/dev/mmcblk1 of=/dev/null bs=1G count=1 &
1+0 records in
1+0 records out
1073741824 bytes (1.1 GB) copied, 4.50898 s, 238 MB/s

So we only want to resort to the thread for the error case.