linux-kernel - Re: [PATCH v2] PCI: endpoint: pci-epf-test: Move DMA check into read/write/copy functions

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Message-ID: <20240820055208.g6iorjl4uhl2jq45@thinkpad>
Date: Tue, 20 Aug 2024 11:22:08 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Rick Wertenbroek <rick.wertenbroek@...il.com>
Cc: rick.wertenbroek@...g-vd.ch, dlemoal@...nel.org,
	alberto.dassatti@...g-vd.ch,
	Krzysztof Wilczyński <kw@...ux.com>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Niklas Cassel <cassel@...nel.org>, Frank Li <Frank.Li@....com>,
	Lars-Peter Clausen <lars@...afoo.de>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PCI: endpoint: pci-epf-test: Move DMA check into
 read/write/copy functions

On Mon, Aug 19, 2024 at 02:01:10PM +0200, Rick Wertenbroek wrote:
> The pci-epf-test PCI endpoint function /drivers/pci/endpoint/function/pci-epf_test.c
> is meant to be used in a PCI endpoint device inside a host computer with
> the host side driver: /drivers/misc/pci_endpoint_test.c.
> 

s/inside/connected to

> The host side driver can request read/write/copy transactions from the
> endpoint function and expects an IRQ from the endpoint function once
> the read/write/copy transaction is finished. These can be issued with or
> without DMA enabled. If the host side driver requests a read/write/copy
> transaction with DMA enabled and the endpoint function does not support
> DMA, the endpoint would only print an error message and wait for further
> commands without sending an IRQ because pci_epf_test_raise_irq() is
> skipped in pci_epf_test_cmd_handler(). This results in the host side
> driver hanging indefinitely waiting for the IRQ.
> 

TBH, it doesn't make sense to control the endpoint DMA from host. Host should
just issue the transfer command, and let the endpoint use DMA or memcpy based on
its capability.

> Move the DMA check into the pci_epf_test_read()/write()/copy() functions
> so that they report a transfer (IO) error and that pci_epf_test_raise_irq()
> is called when a transfer with DMA is requested, even if unsupported.
> 
> The host side driver will no longer hang but report an error on transfer
> (printing "NOT OKAY") thanks to the checksum because no data was moved.
> 
> Signed-off-by: Rick Wertenbroek <rick.wertenbroek@...il.com>
> ---
>  drivers/pci/endpoint/functions/pci-epf-test.c | 29 +++++++++++++++----
>  1 file changed, 23 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/pci/endpoint/functions/pci-epf-test.c b/drivers/pci/endpoint/functions/pci-epf-test.c
> index 7c2ed6eae53a..ec0f79383521 100644
> --- a/drivers/pci/endpoint/functions/pci-epf-test.c
> +++ b/drivers/pci/endpoint/functions/pci-epf-test.c
> @@ -314,6 +314,17 @@ static void pci_epf_test_print_rate(struct pci_epf_test *epf_test,
>  		 (u64)ts.tv_sec, (u32)ts.tv_nsec, rate);
>  }
>  
> +static int pci_epf_test_check_dma(struct pci_epf_test *epf_test,
> +				   struct pci_epf_test_reg *reg)
> +{
> +	if ((READ_ONCE(reg->flags) & FLAG_USE_DMA) &&
> +	    !epf_test->dma_supported) {
> +		dev_err(&epf_test->epf->dev, "DMA transfer not supported\n");
> +		return -EIO;
> +	}
> +	return 0;
> +}
> +
>  static void pci_epf_test_copy(struct pci_epf_test *epf_test,
>  			      struct pci_epf_test_reg *reg)
>  {
> @@ -327,6 +338,10 @@ static void pci_epf_test_copy(struct pci_epf_test *epf_test,
>  	struct device *dev = &epf->dev;
>  	struct pci_epc *epc = epf->epc;
>  
> +	ret = pci_epf_test_check_dma(epf_test, reg);
> +	if (ret)
> +		goto err;
> +
>  	src_addr = pci_epc_mem_alloc_addr(epc, &src_phys_addr, reg->size);
>  	if (!src_addr) {
>  		dev_err(dev, "Failed to allocate source address\n");
> @@ -423,6 +438,10 @@ static void pci_epf_test_read(struct pci_epf_test *epf_test,
>  	struct pci_epc *epc = epf->epc;
>  	struct device *dma_dev = epf->epc->dev.parent;
>  
> +	ret = pci_epf_test_check_dma(epf_test, reg);
> +	if (ret)
> +		goto err;
> +
>  	src_addr = pci_epc_mem_alloc_addr(epc, &phys_addr, reg->size);
>  	if (!src_addr) {
>  		dev_err(dev, "Failed to allocate address\n");
> @@ -507,6 +526,10 @@ static void pci_epf_test_write(struct pci_epf_test *epf_test,
>  	struct pci_epc *epc = epf->epc;
>  	struct device *dma_dev = epf->epc->dev.parent;
>  
> +	ret = pci_epf_test_check_dma(epf_test, reg);
> +	if (ret)
> +		goto err;
> +
>  	dst_addr = pci_epc_mem_alloc_addr(epc, &phys_addr, reg->size);
>  	if (!dst_addr) {
>  		dev_err(dev, "Failed to allocate address\n");
> @@ -647,12 +670,6 @@ static void pci_epf_test_cmd_handler(struct work_struct *work)
>  	WRITE_ONCE(reg->command, 0);
>  	WRITE_ONCE(reg->status, 0);
>  
> -	if ((READ_ONCE(reg->flags) & FLAG_USE_DMA) &&
> -	    !epf_test->dma_supported) {
> -		dev_err(dev, "Cannot transfer data using DMA\n");
> -		goto reset_handler;

Why can't you just set the status and trigger the IRQ here itself? This avoids
duplicating the check inside each command handler.

- Mani

-- 
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