linux-kernel - Re: [PATCH v3 2/3] PCI: endpoint: improve fixed

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Message-ID: <Z_Pw3I2xO7BMSGWW@ryzen>
Date: Mon, 7 Apr 2025 17:35:56 +0200
From: Niklas Cassel <cassel@...nel.org>
To: Jerome Brunet <jbrunet@...libre.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Jon Mason <jdmason@...zu.us>, Dave Jiang <dave.jiang@...el.com>,
	Allen Hubbe <allenbh@...il.com>, Christoph Hellwig <hch@....de>,
	Sagi Grimberg <sagi@...mberg.me>,
	Chaitanya Kulkarni <kch@...dia.com>,
	Marek Vasut <marek.vasut+renesas@...il.com>,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	Yuya Hamamachi <yuya.hamamachi.sx@...esas.com>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	ntb@...ts.linux.dev, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH v3 2/3] PCI: endpoint: improve fixed_size bar handling
 when allocating space

Hello Jerome,

On Mon, Apr 07, 2025 at 04:39:08PM +0200, Jerome Brunet wrote:
> When trying to allocate space for an endpoint function on a BAR with a
> fixed size, the size saved in the 'struct pci_epf_bar' should be the fixed
> size. This is expected by pci_epc_set_bar().
> 
> However, if the fixed_size is smaller that the alignment, the size saved
> in the 'struct pci_epf_bar' matches the alignment and it is a problem for
> pci_epc_set_bar().
> 
> To solve this, continue to allocate space that match the iATU alignment
> requirement but save the size that matches what is present in the BAR.
> 
> Fixes: 2a9a801620ef ("PCI: endpoint: Add support to specify alignment for buffers allocated to BARs")
> Signed-off-by: Jerome Brunet <jbrunet@...libre.com>
> ---
>  drivers/pci/endpoint/pci-epf-core.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/pci/endpoint/pci-epf-core.c b/drivers/pci/endpoint/pci-epf-core.c
> index b7deb0ee1760b23a24f49abf3baf53ea2f273476..fb902b751e1c965c902c5199d57969ae0a757c2e 100644
> --- a/drivers/pci/endpoint/pci-epf-core.c
> +++ b/drivers/pci/endpoint/pci-epf-core.c
> @@ -225,6 +225,7 @@ void pci_epf_free_space(struct pci_epf *epf, void *addr, enum pci_barno bar,
>  	struct device *dev;
>  	struct pci_epf_bar *epf_bar;
>  	struct pci_epc *epc;
> +	size_t size;
>  
>  	if (!addr)
>  		return;
> @@ -237,9 +238,12 @@ void pci_epf_free_space(struct pci_epf *epf, void *addr, enum pci_barno bar,
>  		epf_bar = epf->sec_epc_bar;
>  	}
>  
> +	size = epf_bar[bar].size;
> +	if (epc_features->align)
> +		size = ALIGN(size, epc_features->align);

Personally, I think that you should just save the aligned_size / mem_size /
backing_mem_size as a new struct member, as that avoids the risk that someone
later modifies pci_epf_alloc_space() but forgets to update
pci_epf_free_space() accordingly.

But I will leave the decision to the PCI endpoint maintainers.


Kind regards,
Niklas


> +
>  	dev = epc->dev.parent;
> -	dma_free_coherent(dev, epf_bar[bar].size, addr,
> -			  epf_bar[bar].phys_addr);
> +	dma_free_coherent(dev, size, addr, epf_bar[bar].phys_addr);
>  
>  	epf_bar[bar].phys_addr = 0;
>  	epf_bar[bar].addr = NULL;
> @@ -266,7 +270,7 @@ void *pci_epf_alloc_space(struct pci_epf *epf, size_t size, enum pci_barno bar,
>  			  enum pci_epc_interface_type type)
>  {
>  	u64 bar_fixed_size = epc_features->bar[bar].fixed_size;
> -	size_t align = epc_features->align;
> +	size_t aligned_size, align = epc_features->align;
>  	struct pci_epf_bar *epf_bar;
>  	dma_addr_t phys_addr;
>  	struct pci_epc *epc;
> @@ -287,12 +291,17 @@ void *pci_epf_alloc_space(struct pci_epf *epf, size_t size, enum pci_barno bar,
>  			return NULL;
>  		}
>  		size = bar_fixed_size;
> +	} else {
> +		/* BAR size must be power of two */
> +		size = roundup_pow_of_two(size);
>  	}
>  
> -	if (align)
> -		size = ALIGN(size, align);
> -	else
> -		size = roundup_pow_of_two(size);
> +	/*
> +	 * Allocate enough memory to accommodate the iATU alignment requirement.
> +	 * In most cases, this will be the same as .size but it might be different
> +	 * if, for example, the fixed size of a BAR is smaller than align
> +	 */
> +	aligned_size = align ? ALIGN(size, align) : size;
>  
>  	if (type == PRIMARY_INTERFACE) {
>  		epc = epf->epc;
> @@ -303,7 +312,7 @@ void *pci_epf_alloc_space(struct pci_epf *epf, size_t size, enum pci_barno bar,
>  	}
>  
>  	dev = epc->dev.parent;
> -	space = dma_alloc_coherent(dev, size, &phys_addr, GFP_KERNEL);
> +	space = dma_alloc_coherent(dev, aligned_size, &phys_addr, GFP_KERNEL);
>  	if (!space) {
>  		dev_err(dev, "failed to allocate mem space\n");
>  		return NULL;
> 
> -- 
> 2.47.2
>