Subject: Re: [PATCH net-next v2 2/4] dmaengine: qcom: bam_dma: Add "powered remotely" mode

From: Bjorn Andersson <[email-masked]>
Date: Mon, 11 Oct 2021 10:39:20 -0700

On Mon 11 Oct 07:17 PDT 2021, Stephan Gerhold wrote:

> In some configurations, the BAM DMA controller is set up by a remote
> processor and the local processor can simply start making use of it
> without setting up the BAM. This is already supported using the
> "qcom,controlled-remotely" property.
> 
> However, for some reason another possible configuration is that the
> remote processor is responsible for powering up the BAM, but we are
> still responsible for initializing it (e.g. resetting it etc).
> 
> This configuration is quite challenging to handle properly because
> the power control is handled through separate channels
> (e.g. device-specific SMSM interrupts / smem-states). Great care
> must be taken to ensure the BAM registers are not accessed while
> the BAM is powered off since this results in a bus stall.
> 
> Attempt to support this configuration with minimal device-specific
> code in the bam_dma driver by tracking the number of requested
> channels. Consumers of DMA channels are responsible to only request
> DMA channels when the BAM was powered on by the remote processor,
> and to release them before the BAM is powered off.
> 
> When the first channel is requested the BAM is initialized (reset)
> and it is also put into reset when the last channel was released.
> 
> Signed-off-by: Stephan Gerhold <[email-masked]>
> ---
> Changes since RFC:
>   - Drop qcom-specific terminology "power collapse", instead rename
>     "qcom,remote-power-collapse" -> "qcom,powered-remotely"
> 
> NOTE: This is *not* a compile-time requirement for the BAM-DMUX driver
>       so this could also go through the dmaengine tree.
> 
> See original RFC for a discussion of alternative approaches to handle
> this configuration:
>   https://lore.kernel.org/netdev/[email-masked]/
> ---
>  drivers/dma/qcom/bam_dma.c | 88 ++++++++++++++++++++++++--------------
>  1 file changed, 56 insertions(+), 32 deletions(-)
> 
> diff --git a/drivers/dma/qcom/bam_dma.c b/drivers/dma/qcom/bam_dma.c
> index c8a77b428b52..1b33a3ebbfec 100644
> --- a/drivers/dma/qcom/bam_dma.c
> +++ b/drivers/dma/qcom/bam_dma.c
> @@ -388,6 +388,8 @@ struct bam_device {
>  	/* execution environment ID, from DT */
>  	u32 ee;
>  	bool controlled_remotely;
> +	bool powered_remotely;
> +	u32 active_channels;
>  
>  	const struct reg_offset_data *layout;
>  
> @@ -415,6 +417,44 @@ static inline void __iomem *bam_addr(struct bam_device *bdev, u32 pipe,
>  		r.ee_mult * bdev->ee;
>  }
>  
> +/**
> + * bam_reset - reset and initialize BAM registers

Please include a set of () after the function name.

> + * @bdev: bam device
> + */
> +static void bam_reset(struct bam_device *bdev)
> +{
> +	u32 val;
> +
> +	/* s/w reset bam */
> +	/* after reset all pipes are disabled and idle */
> +	val = readl_relaxed(bam_addr(bdev, 0, BAM_CTRL));
> +	val |= BAM_SW_RST;
> +	writel_relaxed(val, bam_addr(bdev, 0, BAM_CTRL));
> +	val &= ~BAM_SW_RST;
> +	writel_relaxed(val, bam_addr(bdev, 0, BAM_CTRL));

Seems odd to me that we assert and deassert the reset in back-to-back
writes, without any delay etc. That said, this is unrelated to your
patch as you just moved this hunk from below.

> +
> +	/* make sure previous stores are visible before enabling BAM */
> +	wmb();
> +
> +	/* enable bam */
> +	val |= BAM_EN;
> +	writel_relaxed(val, bam_addr(bdev, 0, BAM_CTRL));
> +
> +	/* set descriptor threshhold, start with 4 bytes */
> +	writel_relaxed(DEFAULT_CNT_THRSHLD,
> +			bam_addr(bdev, 0, BAM_DESC_CNT_TRSHLD));
> +
> +	/* Enable default set of h/w workarounds, ie all except BAM_FULL_PIPE */
> +	writel_relaxed(BAM_CNFG_BITS_DEFAULT, bam_addr(bdev, 0, BAM_CNFG_BITS));
> +
> +	/* enable irqs for errors */
> +	writel_relaxed(BAM_ERROR_EN | BAM_HRESP_ERR_EN,
> +			bam_addr(bdev, 0, BAM_IRQ_EN));
> +
> +	/* unmask global bam interrupt */
> +	writel_relaxed(BAM_IRQ_MSK, bam_addr(bdev, 0, BAM_IRQ_SRCS_MSK_EE));
> +}
> +
>  /**
>   * bam_reset_channel - Reset individual BAM DMA channel
>   * @bchan: bam channel
> @@ -512,6 +552,9 @@ static int bam_alloc_chan(struct dma_chan *chan)
>  		return -ENOMEM;
>  	}
>  
> +	if (bdev->active_channels++ == 0 && bdev->powered_remotely)
> +		bam_reset(bdev);
> +
>  	return 0;
>  }
>  
> @@ -565,6 +608,13 @@ static void bam_free_chan(struct dma_chan *chan)
>  	/* disable irq */
>  	writel_relaxed(0, bam_addr(bdev, bchan->id, BAM_P_IRQ_EN));
>  
> +	if (--bdev->active_channels == 0 && bdev->powered_remotely) {
> +		/* s/w reset bam */
> +		val = readl_relaxed(bam_addr(bdev, 0, BAM_CTRL));
> +		val |= BAM_SW_RST;
> +		writel_relaxed(val, bam_addr(bdev, 0, BAM_CTRL));
> +	}
> +
>  err:
>  	pm_runtime_mark_last_busy(bdev->dev);
>  	pm_runtime_put_autosuspend(bdev->dev);
> @@ -1164,38 +1214,10 @@ static int bam_init(struct bam_device *bdev)
>  		bdev->num_channels = val & BAM_NUM_PIPES_MASK;
>  	}
>  
> -	if (bdev->controlled_remotely)
> +	if (bdev->controlled_remotely || bdev->powered_remotely)
>  		return 0;

I think the resulting code would be cleaner if you flipped it around as:

	/* Reset BAM now if fully controlled locally */
	if (!bdev->controlled_remotely && !bdev->powered_remotely)
		bam_reset(bdev);

	return 0;

Regards,
Bjorn

>  
> -	/* s/w reset bam */
> -	/* after reset all pipes are disabled and idle */
> -	val = readl_relaxed(bam_addr(bdev, 0, BAM_CTRL));
> -	val |= BAM_SW_RST;
> -	writel_relaxed(val, bam_addr(bdev, 0, BAM_CTRL));
> -	val &= ~BAM_SW_RST;
> -	writel_relaxed(val, bam_addr(bdev, 0, BAM_CTRL));
> -
> -	/* make sure previous stores are visible before enabling BAM */
> -	wmb();
> -
> -	/* enable bam */
> -	val |= BAM_EN;
> -	writel_relaxed(val, bam_addr(bdev, 0, BAM_CTRL));
> -
> -	/* set descriptor threshhold, start with 4 bytes */
> -	writel_relaxed(DEFAULT_CNT_THRSHLD,
> -			bam_addr(bdev, 0, BAM_DESC_CNT_TRSHLD));
> -
> -	/* Enable default set of h/w workarounds, ie all except BAM_FULL_PIPE */
> -	writel_relaxed(BAM_CNFG_BITS_DEFAULT, bam_addr(bdev, 0, BAM_CNFG_BITS));
> -
> -	/* enable irqs for errors */
> -	writel_relaxed(BAM_ERROR_EN | BAM_HRESP_ERR_EN,
> -			bam_addr(bdev, 0, BAM_IRQ_EN));
> -
> -	/* unmask global bam interrupt */
> -	writel_relaxed(BAM_IRQ_MSK, bam_addr(bdev, 0, BAM_IRQ_SRCS_MSK_EE));
> -
> +	bam_reset(bdev);
>  	return 0;
>  }