linux-kernel - Re: [PATCH v3 5/6] MIPS: implement architecture-specific 'pci_remap

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Message-ID: <88d1835f-cee5-0fb2-f7de-7fc06e8e671d@flygoat.com>
Date:   Thu, 16 Dec 2021 14:27:34 +0000
From:   Jiaxun Yang <jiaxun.yang@...goat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Xi Ruoyao <xry111@...gyan1223.wang>,
        Sergio Paracuellos <sergio.paracuellos@...il.com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Rob Herring <robh@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Liviu Dudau <Liviu.Dudau@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        linux-staging@...ts.linux.dev, NeilBrown <neil@...wn.name>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 5/6] MIPS: implement architecture-specific
 'pci_remap_iospace()'



在 2021/12/16 14:18, Arnd Bergmann 写道:
> On Thu, Dec 16, 2021 at 3:14 PM Jiaxun Yang <jiaxun.yang@...goat.com> wrote:
>> 在 2021/12/16 13:50, Arnd Bergmann 写道:
>>> On Thu, Dec 16, 2021 at 2:07 PM Jiaxun Yang <jiaxun.yang@...goat.com> wrote:
>>>> 在2021年12月16日十二月 上午11:44，Xi Ruoyao写道：
>>>> Another way could be keeping a linked list about PIO->PHYS mapping instead of using the single io_port_base variable.
>>> I think that would add a lot of complexity that isn't needed here. Not
>>> sure if all MIPS CPUs
>>> can do it, but the approach used on Arm is what fits in best with the
>>> PCI drivers, these
>>> reserve a virtual address range for the ports, and ioremap the
>>> physical addresses into
>>> the PIO range according to the mapping.
>> Yes, the Arm way was my previous approach when introducing PCI IO map
>> for Loongson.
>>
>> It got refactored by this patch as TLB entries are expensive on MIPS,
>> also the size of IO range doesn't always fits a page.
> Are PIO accesses common enough that the TLB entry makes a difference?
> I would imagine that on most systems with a PCI bus, there is not even
> a single device that exposes an I/O resource, and even on those devices that
> do, the kernel drivers tend to pick MMIO whenever both are available.

Actually that was claimed by the author of this patch :-)
I can understand the point. As he is working on a ramips system utlizes 
1004Kec,
which has only 32 TLB entries, saving a entry can give considerable 
improvement.

For Loongson as we have legacy i8042/i8259 which can only be accessed via
PIO, the access is very common.

For other systems I guess it's not that common.

Thanks.


>
>        Arnd
- Jiaxun