linux-kernel - Re: [PATCH v4 2/2] PCI: iproc: Add outbound configuration for 32-bit I/O region

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Message-ID: <20190402133851.GA26122@e107981-ln.cambridge.arm.com>
Date:   Tue, 2 Apr 2019 14:38:51 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Srinath Mannam <srinath.mannam@...adcom.com>
Cc:     Ray Jui <ray.jui@...adcom.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Abhishek Shah <abhishek.shah@...adcom.com>
Subject: Re: [PATCH v4 2/2] PCI: iproc: Add outbound configuration for 32-bit
 I/O region

On Tue, Apr 02, 2019 at 04:16:13PM +0530, Srinath Mannam wrote:

[...]

> > Ok - I start to understand. What does it mean in HW terms that your
> > 32bit AXI address region size is 32MB ? Please explain to me in details.
> >
> In our PCIe controller HW, AXI address from 0x42000000 to 0x44000000
> of 32MB size and .
> AXI address from 0x400000000 to 0x480000000 of 2GB size are provided
> to map ob address.
> First IO region is inside 32bit address and second IO region is
> outside 32bit address.
> This code change is to map first IO region(0x42000000 to 0x44000000).
> 
> > IIUC you are using an OARR0 of 128MB in size to map a 32MB address
> > region, that's what I understand this patch does (and the lowest index
> > corresponds to the smallest possible size - it is far from clear by
> > looking at the patch).
> Yes, lowest index corresponds to smallest possible size (128MB).
> In our controller we have multiple windows like OARR0, OARR1, OARR2,
> OARR3 all supports multiple sizes from 128MB to 1024MB.
> These details are given at the top of this driver file, as shown
> below. all windows supports 128MB size still we must use OARR0 window
> to configure first IO region(0x42000000 to 0x44000000).
> 
> static const struct iproc_pcie_ob_map paxb_v2_ob_map[] = {
>         {
>                 /* OARR0/OMAP0 */
>                 .window_sizes = { 128, 256 },
>                 .nr_sizes = 2,
>         },
>         {
>                 /* OARR1/OMAP1 */
>                 .window_sizes = { 128, 256 },
>                 .nr_sizes = 2,
>         },
>         {
>                 /* OARR2/OMAP2 */
>                 .window_sizes = { 128, 256, 512, 1024 },
>                 .nr_sizes = 4,
>         },
>         {
>                 /* OARR3/OMAP3 */
>                 .window_sizes = { 128, 256, 512, 1024 },
>                 .nr_sizes = 4,
>         },
> };

Ok so this patch allows mapping an AXI I/O window that is smaller
than OARR possible sizes, why it was not done from the beginning
I really do not know.

Now explain this to me please:

> This patch add outbound window configuration to map below 32-bit I/O range
> with corresponding PCI memory, which helps to access I/O region in ARM
> 32-bit and one to one mapping of I/O region to PCI memory.
>
> Ex:
> 1. ranges DT property given for current driver is,
>     ranges = <0x83000000 0x0 0x40000000 0x4 0x00000000 0 0x40000000>;
>     I/O region address is 0x400000000
> 2. ranges DT property can be given after this patch,
>     ranges = <0x83000000 0x0 0x42000000 0x0 0x42000000 0 0x2000000>;
>     I/O region address is 0x42000000

Why 1:1 AXI<->PCI address mapping is not possible in (1), how does the
current code works on 32-bit systems and what's the benefit your change
is bringing.

Thanks,
Lorenzo