linux-kernel - Re: [PATCH v2 2/4] PCI: cadence: Use "dma-ranges" instead of "cdns,no-bar-match-nbits" property

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Date:   Mon, 4 May 2020 11:54:24 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Kishon Vijay Abraham I <kishon@...com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Cc:     Tom Joseph <tjoseph@...ence.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Andrew Murray <amurray@...goodpenguin.co.uk>,
        linux-pci@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] PCI: cadence: Use "dma-ranges" instead of
 "cdns,no-bar-match-nbits" property

On 2020-05-04 9:44 am, Kishon Vijay Abraham I wrote:
> Hi Robin,
> 
> On 5/1/2020 9:24 PM, Robin Murphy wrote:
>> On 2020-05-01 3:46 pm, Lorenzo Pieralisi wrote:
>>> [+Robin - to check on dma-ranges intepretation]
>>>
>>> I would need RobH and Robin to review this.
>>>
>>> Also, An ACK from Tom is required - for the whole series.
>>>
>>> On Fri, Apr 17, 2020 at 05:13:20PM +0530, Kishon Vijay Abraham I wrote:
>>>> Cadence PCIe core driver (host mode) uses "cdns,no-bar-match-nbits"
>>>> property to configure the number of bits passed through from PCIe
>>>> address to internal address in Inbound Address Translation register.
>>>>
>>>> However standard PCI dt-binding already defines "dma-ranges" to
>>>> describe the address range accessible by PCIe controller. Parse
>>>> "dma-ranges" property to configure the number of bits passed
>>>> through from PCIe address to internal address in Inbound Address
>>>> Translation register.
>>>>
>>>> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
>>>> ---
>>>>    drivers/pci/controller/cadence/pcie-cadence-host.c | 13 +++++++++++--
>>>>    1 file changed, 11 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/pci/controller/cadence/pcie-cadence-host.c
>>>> b/drivers/pci/controller/cadence/pcie-cadence-host.c
>>>> index 9b1c3966414b..60f912a657b9 100644
>>>> --- a/drivers/pci/controller/cadence/pcie-cadence-host.c
>>>> +++ b/drivers/pci/controller/cadence/pcie-cadence-host.c
>>>> @@ -206,8 +206,10 @@ int cdns_pcie_host_setup(struct cdns_pcie_rc *rc)
>>>>        struct device *dev = rc->pcie.dev;
>>>>        struct platform_device *pdev = to_platform_device(dev);
>>>>        struct device_node *np = dev->of_node;
>>>> +    struct of_pci_range_parser parser;
>>>>        struct pci_host_bridge *bridge;
>>>>        struct list_head resources;
>>>> +    struct of_pci_range range;
>>>>        struct cdns_pcie *pcie;
>>>>        struct resource *res;
>>>>        int ret;
>>>> @@ -222,8 +224,15 @@ int cdns_pcie_host_setup(struct cdns_pcie_rc *rc)
>>>>        rc->max_regions = 32;
>>>>        of_property_read_u32(np, "cdns,max-outbound-regions", &rc->max_regions);
>>>>    -    rc->no_bar_nbits = 32;
>>>> -    of_property_read_u32(np, "cdns,no-bar-match-nbits", &rc->no_bar_nbits);
>>>> +    if (!of_pci_dma_range_parser_init(&parser, np))
>>>> +        if (of_pci_range_parser_one(&parser, &range))
>>>> +            rc->no_bar_nbits = ilog2(range.size);
>>
>> You probably want "range.pci_addr + range.size" here just in case the bottom of
>> the window is ever non-zero. Is there definitely only ever a single inbound
>> window to consider?
> 
> Cadence IP has 3 inbound address translation registers, however we use only 1
> inbound address translation register to map the entire 32 bit or 64 bit address
> region.

OK, if anything that further strengthens the argument for deprecating a 
single "number of bits" property in favour of ranges that accurately 
describe the window(s). However it also suggests that other users in 
future might have some expectation that specifying "dma-ranges" with up 
to 3 entries should work to allow a more restrictive inbound 
configuration. Thus it would be desirable to make the code a little more 
robust here - even if we don't support multiple windows straight off, it 
would still be better to implement it in a way that can be cleanly 
extended later, and at least say something if more ranges are specified 
rather than just silently ignoring them.

>> I believe that pci_parse_request_of_pci_ranges() could do the actual parsing
>> for you, but I suppose plumbing that in plus processing the resulting
>> dma_ranges resource probably ends up a bit messier than the concise open-coding
>> here.
> 
> right, pci_parse_request_of_pci_ranges() parses "ranges" property and is used
> for outbound configuration, whereas here we parse "dma-ranges" property and is
> used for inbound configuration.

If you give it a valid third argument it *also* parses "dma-ranges" into 
a list of inbound regions. This is already used by various other drivers 
for equivalent inbound window setup, which is what I was hinting at 
before, but given the extensibility argument above I'm now going to 
actively suggest following that pattern for consistency.

Robin.