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

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Message-ID: <1jjz7wvuyj.fsf@starbuckisacylon.baylibre.com>
Date: Mon, 07 Apr 2025 17:43:00 +0200
From: Jerome Brunet <jbrunet@...libre.com>
To: Niklas Cassel <cassel@...nel.org>
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

On Mon 07 Apr 2025 at 17:35, Niklas Cassel <cassel@...nel.org> wrote:

> 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.

I tried but it looked a bit silly to store that when it was only a
matter of calling ALIGN() with parameters we already had, and it is
supposed to be only used in those two functions.

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

Ultimately, I do not have a strong opinion on this. Either way is fine
by me.

>
> 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
>> 

-- 
Jerome