linux-kernel - Re: [PATCH] soundwire: cadence: add paranoid check on self-clearing bits

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Date:   Mon, 2 Aug 2021 09:20:20 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Bard Liao <yung-chuan.liao@...ux.intel.com>
Cc:     alsa-devel@...a-project.org, linux-kernel@...r.kernel.org,
        gregkh@...uxfoundation.org, srinivas.kandagatla@...aro.org,
        rander.wang@...ux.intel.com, pierre-louis.bossart@...ux.intel.com,
        sanyog.r.kale@...el.com, bard.liao@...el.com
Subject: Re: [PATCH] soundwire: cadence: add paranoid check on self-clearing
 bits

On 14-07-21, 13:13, Bard Liao wrote:
> From: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
> 
> The Cadence IP exposes a small number of self-clearing bits in
> the MCP_CONTROL and MCP_CONFIG_UPDATE registers.
> 
> We currently do not check that those bits are indeed cleared,
> e.g. during resume operations. That could lead to resuming peripheral
> devices too early.
> 
> In addition, if we happen to read these registers, update one of the
> fields and write the register back, we may be writing stale data that
> might have been cleared in hardware. These sort of race conditions
> could lead to e.g. doing a hw_reset twice or stopping a clock that
> just restarted. There is no clear way of avoiding these potential race
> conditions other than making sure that these registers fields are
> cleared before any read-modify-write sequence. If we detect this sort
> of errors, we only log them since there is no clear recovery
> possible. The only way out is likely to restart the IP with a
> suspend/resume cycle.
> 
> Note that the checks are performed before updating the registers, as
> well as after the Intel 'sync go' sequence in multi-link mode. That
> should cover both the start and end of suspend/resume hardware
> configurations. The Multi-Master mode gates the configuration updates
> until the 'sync go' signal is asserted, so we only check on init and
> after the end of the 'sync go' sequence.
> 
> The duration of the usleep_range() was defined by the GSYNC frequency
> used in multi-master mode. With a 4kHz frequency, any configuration
> change might be deferred by up to 250us. Extending the range to
> 1000-1500us should guarantee that the configuration change is
> completed without any significant impact on the overall resume
> time.

There were some checkpatch warns, but I think code will looks worse if
we split lines up, so applied now

-- 
~Vinod