Re: [PATCH] PCI: mediatek-gen3: Fix translation window

[Jianjun Wang (王建军)]

On Wed, 2023-10-11 at 17:38 +0200, Alexandre Mergnat wrote:
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> On 11/10/2023 14:26, Jianjun Wang wrote:
> > The size of translation table should be a power of 2, using fls()
> cannot 
> > get the proper value when the size is not a power of 2. For
> example, 
> > fls(0x3e00000) - 1 = 25, hence the PCIe translation window size
> will be 
> > set to 0x2000000 instead of the expected size 0x3e00000. Fix
> translation 
> > window by splitting the MMIO space to multiple tables if its size
> is not 
> > a power of 2.
> 
> Hi Jianjun,
> 
> I've no knowledge in PCIE, so maybe what my suggestion is stupid:
> 
> Is it mandatory to fit the translation table size with 0x3e00000 (in 
> this example) ?
> I'm asking because you can have an issue by reaching the maximum 
> translation table number.
> 
> Is it possible to just use only one table with the power of 2 size
> above 
> 0x3e00000 => 0x4000000 ( fls(0x3e00000) = 26 = 0x4000000). The
> downside 
> of this method is wasting allocation space. AFAIK I already see this 
> kind of method for memory protection/allocation in embedded systems,
> so 
> I'm wondering if this method is safer than using multiple table for
> only 
> one size which isn't a power of 2.

Hi Alexandre,

It's not mandatory to fit the translation table size with 0x3e00000,
and yes we can use only one table with the power of 2 size to prevent
this.

For MediaTek's SoCs, the MMIO space range for each PCIe port is fixed,
and it will always be a power of 2, most of them will be 64MB. The
reason we have the size which isn't a power of 2 is that we reserve an
IO space for compatible purpose, some older devices may still use IO
space.

Take MT8195 as an example, its MMIO size is 64MB, and the declaration
in the DT is like:
ranges = <0x81000000 0 0x20000000 0x0 0x20000000 0 0x200000>,
         <0x82000000 0 0x20200000 0x0 0x20200000 0 0x3e00000>;

The MMIO space is splited to 2MB IO space and 62MB MEM space, that's
cause the current risk of the MEM space range, its actual available MEM
space is 32MB. But it still works for now because most of the devices
only require a very small amount of MEM space and will not reach ranges
higher than 32MB.

So for the concern of reaching the maximum translation table number, I
think maybe we can just print the warning message instead of return
error code, since it still works but have some limitations(MEM space
not set as DT expected).

Thanks.
> 
> 
> -- 
> Regards,
> Alexandre