linux-kernel - Re: [PATCH] i2c: designware: Introduce cooldown timer to AMDPSP driver

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Message-ID: <CAOtMz3P94+Mmwx7oD=rEC2V0Cbd+NGAczfSdKNuxu8w+4FU1sA@mail.gmail.com>
Date:   Tue, 9 Aug 2022 09:41:36 +0200
From:   Jan Dąbroś <jsd@...ihalf.com>
To:     linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
        jarkko.nikula@...ux.intel.com, andriy.shevchenko@...ux.intel.com
Cc:     mika.westerberg@...ux.intel.com, wsa@...nel.org,
        rrangel@...omium.org, mw@...ihalf.com, upstream@...ihalf.com
Subject: Re: [PATCH] i2c: designware: Introduce cooldown timer to AMDPSP driver

Hi Jarko, Andriy,

Could you please take a look?

Best Regards,
Jan


pon., 25 lip 2022 o 10:03 Jan Dabros <jsd@...ihalf.com> napisał(a):
>
> In order to optimize performance, limit amount of back and forth
> transactions between x86 and PSP. This is done by introduction of
> cooldown period - that is window in which x86 isn't releasing the bus
> immediately after each I2C transaction.
>
> In order to protect PSP from being starved while waiting for
> arbitration, after a programmed time bus is automatically released by a
> deferred function.
>
> Signed-off-by: Jan Dabros <jsd@...ihalf.com>
> ---
>  drivers/i2c/busses/i2c-designware-amdpsp.c | 68 +++++++++++++++++-----
>  1 file changed, 53 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-designware-amdpsp.c b/drivers/i2c/busses/i2c-designware-amdpsp.c
> index b624356c945f..2e1bb5ae72c3 100644
> --- a/drivers/i2c/busses/i2c-designware-amdpsp.c
> +++ b/drivers/i2c/busses/i2c-designware-amdpsp.c
> @@ -6,6 +6,7 @@
>  #include <linux/io-64-nonatomic-lo-hi.h>
>  #include <linux/psp-sev.h>
>  #include <linux/types.h>
> +#include <linux/workqueue.h>
>
>  #include <asm/msr.h>
>
> @@ -15,6 +16,8 @@
>  #define PSP_MBOX_OFFSET                0x10570
>  #define PSP_CMD_TIMEOUT_US     (500 * USEC_PER_MSEC)
>
> +#define PSP_I2C_COOLDOWN_TIME_MS 100
> +
>  #define PSP_I2C_REQ_BUS_CMD            0x64
>  #define PSP_I2C_REQ_RETRY_CNT          400
>  #define PSP_I2C_REQ_RETRY_DELAY_US     (25 * USEC_PER_MSEC)
> @@ -240,6 +243,42 @@ static int psp_send_i2c_req(enum psp_i2c_req_type i2c_req_type)
>         return ret;
>  }
>
> +static void release_bus_now(void)
> +{
> +       int status;
> +
> +       if (!psp_i2c_sem_acquired)
> +               return;
> +
> +       status = psp_send_i2c_req(PSP_I2C_REQ_RELEASE);
> +       if (status)
> +               return;
> +
> +       dev_dbg(psp_i2c_dev, "PSP semaphore held for %ums\n",
> +               jiffies_to_msecs(jiffies - psp_i2c_sem_acquired));
> +
> +       psp_i2c_sem_acquired = 0;
> +}
> +
> +static void psp_release_i2c_bus_deferred(struct work_struct *work)
> +{
> +
> +       mutex_lock(&psp_i2c_access_mutex);
> +
> +       /*
> +        * If there is any pending transaction, cannot release the bus here.
> +        * psp_release_i2c_bus will take care of this later.
> +        */
> +       if (psp_i2c_access_count)
> +               goto cleanup;
> +
> +       release_bus_now();
> +
> +cleanup:
> +       mutex_unlock(&psp_i2c_access_mutex);
> +}
> +static DECLARE_DELAYED_WORK(release_queue, psp_release_i2c_bus_deferred);
> +
>  static int psp_acquire_i2c_bus(void)
>  {
>         int status;
> @@ -250,21 +289,23 @@ static int psp_acquire_i2c_bus(void)
>         if (psp_i2c_mbox_fail)
>                 goto cleanup;
>
> +       psp_i2c_access_count++;
> +
>         /*
> -        * Simply increment usage counter and return if PSP semaphore was
> -        * already taken by kernel.
> +        * No need to request bus arbitration once we are inside cooldown
> +        * period.
>          */
> -       if (psp_i2c_access_count) {
> -               psp_i2c_access_count++;
> +       if (psp_i2c_sem_acquired)
>                 goto cleanup;
> -       }
>
>         status = psp_send_i2c_req(PSP_I2C_REQ_ACQUIRE);
>         if (status)
>                 goto cleanup;
>
>         psp_i2c_sem_acquired = jiffies;
> -       psp_i2c_access_count++;
> +
> +       schedule_delayed_work(&release_queue,
> +                             msecs_to_jiffies(PSP_I2C_COOLDOWN_TIME_MS));
>
>         /*
>          * In case of errors with PSP arbitrator psp_i2c_mbox_fail variable is
> @@ -279,8 +320,6 @@ static int psp_acquire_i2c_bus(void)
>
>  static void psp_release_i2c_bus(void)
>  {
> -       int status;
> -
>         mutex_lock(&psp_i2c_access_mutex);
>
>         /* Return early if mailbox was malfunctional */
> @@ -295,13 +334,12 @@ static void psp_release_i2c_bus(void)
>         if (psp_i2c_access_count)
>                 goto cleanup;
>
> -       /* Send a release command to PSP */
> -       status = psp_send_i2c_req(PSP_I2C_REQ_RELEASE);
> -       if (status)
> -               goto cleanup;
> -
> -       dev_dbg(psp_i2c_dev, "PSP semaphore held for %ums\n",
> -               jiffies_to_msecs(jiffies - psp_i2c_sem_acquired));
> +       /*
> +        * Send a release command to PSP if the cooldown timeout elapsed but x86 still
> +        * owns the ctrlr.
> +        */
> +       if (!delayed_work_pending(&release_queue))
> +               release_bus_now();
>
>  cleanup:
>         mutex_unlock(&psp_i2c_access_mutex);
> --
> 2.37.1.359.gd136c6c3e2-goog
>