linux-kernel - Re: [PATCH v13 5/6] i2c: tegra: Add support for SW mutex register

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Message-ID: <tqkqtn7nwbc2pmveymrea5fdzrkl6q3wuehy7kdkgb6kcze4wf@r6a7sug7wnfv>
Date: Thu, 27 Nov 2025 10:36:21 +0100
From: Thierry Reding <thierry.reding@...il.com>
To: Akhil R <akhilrajeev@...dia.com>
Cc: andi.shyti@...nel.org, digetx@...il.com, jonathanh@...dia.com, 
	linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org, 
	wsa+renesas@...g-engineering.com, wsa@...nel.org, kkartik@...dia.com, ldewangan@...dia.com, 
	smangipudi@...dia.com
Subject: Re: [PATCH v13 5/6] i2c: tegra: Add support for SW mutex register

On Tue, Nov 18, 2025 at 07:36:19PM +0530, Akhil R wrote:
> From: Kartik Rajput <kkartik@...dia.com>
> 
> Add support for SW mutex register introduced in Tegra264 to provide
> an option to share the interface between multiple firmwares and/or
> VMs. This involves following steps:
> 
>  - A firmware/OS writes its unique ID to the mutex REQUEST field.
>  - Ownership is established when reading the GRANT field returns the
>    same ID.
>  - If GRANT shows a different non-zero ID, the firmware/OS retries
>    until timeout.
>  - After completing access, it releases the mutex by writing 0.
> 
> However, the hardware does not ensure any protection based on the
> values. The driver/firmware should honor the peer who already holds
> the mutex.
> 
> Signed-off-by: Kartik Rajput <kkartik@...dia.com>
> Signed-off-by: Akhil R <akhilrajeev@...dia.com>
> Reviewed-by: Jon Hunter <jonathanh@...dia.com>
> ---
> v7 -> v8:
>         * Use `bool` instead of `int` for `locked` variable in
>           tegra_i2c_mutex_lock() function.
> v6 -> v7:
>         * Return bool from tegra_i2c_mutex_acquired() and
>           tegra_i2c_mutex_trylock() functions.
>         * Move `has_mutex` check inside tegra_i2c_mutex_lock/unlock
>           functions.
>         * Remove redundant empty line added in tegra_i2c_xfer() in v6.
>         * Fix pm_runtime_put() not getting called if mutex unlock fails.
>         * In tegra_i2c_mutex_lock() simplify the logic to check if the
>           mutex is acquired or not by checking the value of `ret`
>           variable.
>         * Update commit message to describe the functioning of SW mutex
>           feature.
> v4 -> v6:
>         * Guard tegra_i2c_mutex_lock() and tegra_i2c_mutex_unlock() to
>           ensure that they are called on platforms which support SW
>           mutex.
> v3 -> v4:
>         * Update timeout logic of tegra_i2c_mutex_lock() to use
>           read_poll_timeout APIs for improving timeout logic.
>         * Add tegra_i2c_mutex_acquired() to check if mutex is acquired
>           or not.
>         * Rename I2C_SW_MUTEX_ID as I2C_SW_MUTEX_ID_CCPLEX.
>         * Function tegra_i2c_poll_register() was moved unnecessarily, it
>           has now been moved to its original location.
>         * Use tegra_i2c_mutex_lock/unlock APIs in the tegra_i2c_xfer()
>           function. This ensures proper propagation of error in case
>           mutex lock fails.
>           Please note that as the function tegra_i2c_xfer() is
>           already guarded by the bus lock operation there is no need of
>           additional lock for the tegra_i2c_mutex_lock/unlock APIs.
> v2 -> v3:
>         * Update tegra_i2c_mutex_trylock and tegra_i2c_mutex_unlock to
>           use readl and writel APIs instead of i2c_readl and i2c_writel
>           which use relaxed APIs.
>         * Use dev_warn instead of WARN_ON if mutex lock/unlock fails.
> v1 -> v2:
>         * Fixed typos.
>         * Fix tegra_i2c_mutex_lock() logic.
>         * Add a timeout in tegra_i2c_mutex_lock() instead of polling for
>           mutex indefinitely.
> ---
>  drivers/i2c/busses/i2c-tegra.c | 93 ++++++++++++++++++++++++++++++++++
>  1 file changed, 93 insertions(+)

Acked-by: Thierry Reding <treding@...dia.com>

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