linux-kernel - Re: [PATCH v3 1/5] PCI: dwc: Refactor dw_pcie_edma_find

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Date: Mon, 26 Feb 2024 15:45:16 +0300
From: Serge Semin <fancer.lancer@...il.com>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc: Siddharth Vadapalli <s-vadapalli@...com>, 
	Jingoo Han <jingoohan1@...il.com>, Gustavo Pimentel <gustavo.pimentel@...opsys.com>, 
	Lorenzo Pieralisi <lpieralisi@...nel.org>, Krzysztof Wilczyński <kw@...ux.com>, 
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	Marek Vasut <marek.vasut+renesas@...il.com>, Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>, 
	Kishon Vijay Abraham I <kishon@...nel.org>, linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-renesas-soc@...r.kernel.org, linux-arm-msm@...r.kernel.org, mhi@...ts.linux.dev
Subject: Re: [PATCH v3 1/5] PCI: dwc: Refactor dw_pcie_edma_find_chip() API

Hi Manivannan

On Mon, Feb 26, 2024 at 05:07:26PM +0530, Manivannan Sadhasivam wrote:
> In order to add support for Hyper DMA (HDMA), let's refactor the existing
> dw_pcie_edma_find_chip() API by moving the common code to separate
> functions.
> 
> No functional change.
> 
> Suggested-by: Serge Semin <fancer.lancer@...il.com>
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> ---
>  drivers/pci/controller/dwc/pcie-designware.c | 52 +++++++++++++++++++++-------
>  1 file changed, 39 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-designware.c b/drivers/pci/controller/dwc/pcie-designware.c
> index 250cf7f40b85..193fcd86cf93 100644
> --- a/drivers/pci/controller/dwc/pcie-designware.c
> +++ b/drivers/pci/controller/dwc/pcie-designware.c
> @@ -880,7 +880,17 @@ static struct dw_edma_plat_ops dw_pcie_edma_ops = {
>  	.irq_vector = dw_pcie_edma_irq_vector,
>  };
>  
> -static int dw_pcie_edma_find_chip(struct dw_pcie *pci)
> +static void dw_pcie_edma_init_data(struct dw_pcie *pci)
> +{
> +	pci->edma.dev = pci->dev;
> +
> +	if (!pci->edma.ops)
> +		pci->edma.ops = &dw_pcie_edma_ops;
> +
> +	pci->edma.flags |= DW_EDMA_CHIP_LOCAL;
> +}
> +
> +static int dw_pcie_edma_find_mf(struct dw_pcie *pci)
>  {
>  	u32 val;
>  
> @@ -900,24 +910,27 @@ static int dw_pcie_edma_find_chip(struct dw_pcie *pci)
>  	else
>  		val = dw_pcie_readl_dbi(pci, PCIE_DMA_VIEWPORT_BASE + PCIE_DMA_CTRL);
> 

> -	if (val == 0xFFFFFFFF && pci->edma.reg_base) {
> -		pci->edma.mf = EDMA_MF_EDMA_UNROLL;
> -
> -		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
> -	} else if (val != 0xFFFFFFFF) {
> -		pci->edma.mf = EDMA_MF_EDMA_LEGACY;
> +	/* Set default mapping format here and update it below if needed */
> +	pci->edma.mf = EDMA_MF_EDMA_LEGACY;
>  
> +	if (val == 0xFFFFFFFF && pci->edma.reg_base)
> +		pci->edma.mf = EDMA_MF_EDMA_UNROLL;
> +	else if (val != 0xFFFFFFFF)
>  		pci->edma.reg_base = pci->dbi_base + PCIE_DMA_VIEWPORT_BASE;
> -	} else {
> +	else
>  		return -ENODEV;
> -	}

Sorry for not posting my opinion about this earlier, but IMO v2 code
was more correct than this one. This version makes the code being not
linear as it was in v2, thus harder to comprehend:

1. Setting up a default value and then overriding it or not makes the
reader to keep in mind the initialized value which is harder than to
just read what is done in the respective branch.

2. Splitting up the case clause with respective inits and the mapping
format setting up also makes it harder to comprehend what's going on.
In the legacy case the reg-base address and the mapping format init are
split up while they should have been done simultaneously only if (val
!= 0xFFFFFFFF).

3. The most of the current devices has the unrolled mapping (available
since v4.9 IP-core), thus having the mf field pre-initialized produces
a redundant store operation for the most of the modern devices.

4. Getting rid from the curly braces isn't something what should be
avoided at any cost and doesn't give any optimization really. It
doesn't cause having less C-lines of the source code and doesn't
improve the code readability.

So to speak, I'd suggest to get back the v2 implementation here.

>  
> -	pci->edma.dev = pci->dev;
> +	return 0;
> +}
>  
> -	if (!pci->edma.ops)
> -		pci->edma.ops = &dw_pcie_edma_ops;
> +static int dw_pcie_edma_find_channels(struct dw_pcie *pci)
> +{
> +	u32 val;
>  
> -	pci->edma.flags |= DW_EDMA_CHIP_LOCAL;

> +	if (pci->edma.mf == EDMA_MF_EDMA_LEGACY)
> +		val = dw_pcie_readl_dbi(pci, PCIE_DMA_VIEWPORT_BASE + PCIE_DMA_CTRL);
> +	else
> +		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);

Just dw_pcie_readl_dma(pci, PCIE_DMA_CTRL)

-Serge(y)

>  
>  	pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
>  	pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
> @@ -930,6 +943,19 @@ static int dw_pcie_edma_find_chip(struct dw_pcie *pci)
>  	return 0;
>  }
>  
> +static int dw_pcie_edma_find_chip(struct dw_pcie *pci)
> +{
> +	int ret;
> +
> +	dw_pcie_edma_init_data(pci);
> +
> +	ret = dw_pcie_edma_find_mf(pci);
> +	if (ret)
> +		return ret;
> +
> +	return dw_pcie_edma_find_channels(pci);
> +}
> +
>  static int dw_pcie_edma_irq_verify(struct dw_pcie *pci)
>  {
>  	struct platform_device *pdev = to_platform_device(pci->dev);
> 
> -- 
> 2.25.1
>