[PATCH v2] i2c: mediatek: Optimize master_xfer() and avoid circular locking

[Qii Wang]

On Mon, 2022-04-11 at 15:21 +0200, AngeloGioacchino Del Regno wrote:
> Especially (but not only) during probe, it may happen that multiple
> devices are communicating via i2c (or multiple i2c busses) and
> sometimes while others are probing asynchronously.
> For example, a Cr50 TPM may be filling entropy (or userspace may be
> reading random data) while the rt5682 (i2c) codec driver reads/sets
> some registers, like while getting/setting a clock's rate, which
> happens both during probe and during system operation.
> 
> In this driver, the mtk_i2c_transfer() function (which is the i2c
> .master_xfer() callback) was granularly managing the clocks by
> performing a clk_bulk_prepare_enable() to start them and its inverse.
> This is not only creating possible circular locking dependencies in
> the some cases (like former explanation), but it's also suboptimal,
> as clk_core prepare/unprepare operations are using mutex locking,
> which creates a bit of unwanted overhead (for example, i2c trackpads
> will call master_xfer() every few milliseconds!).
> 
> With this commit, we avoid both the circular locking and additional
> overhead by changing how we handle the clocks in this driver:
> - Prepare the clocks during probe (and PM resume)
> - Enable/disable clocks in mtk_i2c_transfer()
> - Unprepare the clocks only for driver removal (and PM suspend)
> 
> For the sake of providing a full explanation: during probe, the
> clocks are not only prepared but also enabled, as this is needed
> for some hardware initialization but, after that, we are disabling
> but not unpreparing them, leaving an expected state for the
> aforementioned clock handling strategy.
> 
> Signed-off-by: AngeloGioacchino Del Regno <
> angelogioacchino.delregno at collabora.com>
> Tested-by: Nícolas F. R. A. Prado <nfraprado at collabora.com>

Reviewed-by: Qii Wang <qii.wang at mediatek.com>

> ---
> 
> v2: Fixed typos in commit description
> 
>  drivers/i2c/busses/i2c-mt65xx.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
>