Re: [PATCH 3/3] hwrng: imx-rngc - use polling to wait for end of seeding

Alexander Stein
2023-08-23T05:33:56Z

Am Dienstag, 22. August 2023, 17:25:53 CEST schrieb Martin Kaiser:
> Use polling to wait until the imx-rngc is properly seeded.

What is the benefit of burning CPU cycles while waiting for hardware?

> We do this only in the init function when the imx-rngc becomes active.
> Polling is ok at this time, there's nothing else we could do while
> we're waiting.
>
> We can now remove the code for the interrupt and the completion.

Please split the change to polling and IRQ removal into two patches.

> Signed-off-by: Martin Kaiser <martin@kaiser.cx>
> ---
> drivers/char/hw_random/imx-rngc.c | 81 ++++---------------------------
> 1 file changed, 9 insertions(+), 72 deletions(-)
>
> diff --git a/drivers/char/hw_random/imx-rngc.c
> b/drivers/char/hw_random/imx-rngc.c index d2df468fd460..7ab9aada72d0 100644
> --- a/drivers/char/hw_random/imx-rngc.c
> +++ b/drivers/char/hw_random/imx-rngc.c
> @@ -63,12 +63,6 @@ struct imx_rngc {
> struct clk *clk;
> void __iomem *base;
> struct hwrng rng;
> - struct completion rng_op_done;
> - /*
> - * err_reg is written only by the irq handler and read only
> - * when interrupts are masked, we need no spinlock
> - */
> - u32 err_reg;
> };
>
>
> @@ -91,15 +85,6 @@ static inline void imx_rngc_irq_mask_clear(struct
> imx_rngc *rngc) writel(cmd, rngc->base + RNGC_COMMAND);
> }
>
> -static inline void imx_rngc_irq_unmask(struct imx_rngc *rngc)
> -{
> - u32 ctrl;
> -
> - ctrl = readl(rngc->base + RNGC_CONTROL);
> - ctrl &= ~(RNGC_CTRL_MASK_DONE | RNGC_CTRL_MASK_ERROR);
> - writel(ctrl, rngc->base + RNGC_CONTROL);
> -}
> -
> static int imx_rngc_self_test(struct imx_rngc *rngc)
> {
> u32 cmd, status;
> @@ -143,56 +128,32 @@ static int imx_rngc_read(struct hwrng *rng, void
> *data, size_t max, bool wait) return retval ? retval : -EIO;
> }
>
> -static irqreturn_t imx_rngc_irq(int irq, void *priv)
> -{
> - struct imx_rngc *rngc = (struct imx_rngc *)priv;
> - u32 status;
> -
> - /*
> - * clearing the interrupt will also clear the error register
> - * read error and status before clearing
> - */
> - status = readl(rngc->base + RNGC_STATUS);
> - rngc->err_reg = readl(rngc->base + RNGC_ERROR);
> -
> - imx_rngc_irq_mask_clear(rngc);
> -
> - if (status & (RNGC_STATUS_SEED_DONE | RNGC_STATUS_ST_DONE))
> - complete(&rngc->rng_op_done);
> -
> - return IRQ_HANDLED;
> -}
> -
> static int imx_rngc_init(struct hwrng *rng)
> {
> struct imx_rngc *rngc = container_of(rng, struct imx_rngc, rng);
> - u32 cmd, ctrl;
> + u32 cmd, ctrl, status, err_reg;
> int ret;
>
> /* clear error */
> cmd = readl(rngc->base + RNGC_COMMAND);
> writel(cmd | RNGC_CMD_CLR_ERR, rngc->base + RNGC_COMMAND);
>
> - imx_rngc_irq_unmask(rngc);
> -
> /* create seed, repeat while there is some statistical error */
> do {
> /* seed creation */
> cmd = readl(rngc->base + RNGC_COMMAND);
> writel(cmd | RNGC_CMD_SEED, rngc->base + RNGC_COMMAND);
>
> - ret = wait_for_completion_timeout(&rngc->rng_op_done,
> msecs_to_jiffies(RNGC_TIMEOUT)); - if (!ret) {
> - ret = -ETIMEDOUT;
> - goto err;
> - }
> + ret = readl_poll_timeout(rngc->base + RNGC_STATUS, status,
> + status &
RNGC_STATUS_SEED_DONE, 1000, RNGC_TIMEOUT * 1000);

So you want to poll for up to 3s?

Best regards,
Alexander

> + if (ret < 0)
> + return ret;
>
> - } while (rngc->err_reg == RNGC_ERROR_STATUS_STAT_ERR);
> + err_reg = readl(rngc->base + RNGC_ERROR);
> + } while (err_reg == RNGC_ERROR_STATUS_STAT_ERR);
>
> - if (rngc->err_reg) {
> - ret = -EIO;
> - goto err;
> - }
> + if (err_reg)
> + return -EIO;
>
> /*
> * enable automatic seeding, the rngc creates a new seed
automatically
> @@ -202,23 +163,7 @@ static int imx_rngc_init(struct hwrng *rng)
> ctrl |= RNGC_CTRL_AUTO_SEED;
> writel(ctrl, rngc->base + RNGC_CONTROL);
>
> - /*
> - * if initialisation was successful, we keep the interrupt
> - * unmasked until imx_rngc_cleanup is called
> - * we mask the interrupt ourselves if we return an error
> - */
> return 0;
> -
> -err:
> - imx_rngc_irq_mask_clear(rngc);
> - return ret;
> -}
> -
> -static void imx_rngc_cleanup(struct hwrng *rng)
> -{
> - struct imx_rngc *rngc = container_of(rng, struct imx_rngc, rng);
> -
> - imx_rngc_irq_mask_clear(rngc);
> }
>
> static int __init imx_rngc_probe(struct platform_device *pdev)
> @@ -254,12 +199,9 @@ static int __init imx_rngc_probe(struct platform_device
> *pdev) if (rng_type != RNGC_TYPE_RNGC && rng_type != RNGC_TYPE_RNGB)
> return -ENODEV;
>
> - init_completion(&rngc->rng_op_done);
> -
> rngc->rng.name = pdev->name;
> rngc->rng.init = imx_rngc_init;
> rngc->rng.read = imx_rngc_read;
> - rngc->rng.cleanup = imx_rngc_cleanup;
> rngc->rng.quality = 19;
>
> rngc->dev = &pdev->dev;
> @@ -267,11 +209,6 @@ static int __init imx_rngc_probe(struct platform_device
> *pdev)
>
> imx_rngc_irq_mask_clear(rngc);
>
> - ret = devm_request_irq(&pdev->dev,
> - irq, imx_rngc_irq, 0, pdev->name, (void *)rngc);
> - if (ret)
> - return dev_err_probe(&pdev->dev, ret, "Can't get interrupt
working.\n");
> -
> if (self_test) {
> ret = imx_rngc_self_test(rngc);
> if (ret)

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