[PATCH v1 09/10] pci: Add driver for Broadcom STB PCIe controller

[Marek Szyprowski]

Hi Nicolas,

On 27.04.2020 12:15, Nicolas Saenz Julienne wrote:
> On Fri, 2020-04-24 at 18:50 +0200, Sylwester Nawrocki wrote:
>> This patch adds basic driver for the Broadcom STB PCIe host controller.
>> The code is based on Linux upstream driver (pcie-brcmstb.c) with MSI
>> handling removed. The inbound access memory region is not currently
>> parsed from dma-ranges DT property and a fixed 4GB region is used.
>>
>> The patch has been tested on RPI4 board, i.e. on BCM2711 SoC with VL805
>> USB Host Controller.
>>
>> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne at suse.de>
>> Signed-off-by: Sylwester Nawrocki <s.nawrocki at samsung.com>
>> ---
>> Changes since RFC:
>>   - reworked to align with current Linux mainline version and u-boot driver
>>     by Nicolas Saenz Julienne
> [...]
>
>> +
>> +	/*
>> +	 * TODO: Use the base address and size(s) provided in the dma-ranges
>> +	 * property.
>> +	 */
>> +	rc_bar2_offset = 0;
>> +	rc_bar2_size = 1ULL << 32;
>  From experience this works fine, although it highly depends on how DMA memory is
> handled in u-boot.
>
> For example, in arm64 Linux, DMA memory was allocated from ZONE_DMA32, which
> would return memory smaller than 4GB. This was not good enough for bcm2711b0 --
> revision b0 of rpi4's SoC, so far the most common out there -- which has an
> internal wiring bug that prevents PCIe from accessing memory higher than 3GB
> (RPi4 might have up to 4GB).  So we had to introduce a ZONE_DMA, which covers
> the lower GB of memory, in order to allocate suitable DMA memory for PCI and
> other DMA limited devices.
>
> While playing around with u-boot's xHCI I saw that all memory is allocated is
> located in the lower 1GB. But never got to look into why. I'm curious to know
> if someone knows how's that handled in u-boot. Ultimately, depending on how it
> works, we might be able to just disregard dma-ranges altogether.
>
> For some extra context xhci_malloc() gets its memory from memalign(). And it's
> not clear to me how that function decides which memory to use.

I think that memalign() allocates memory from the uboot's defined SDRAM 
(from its end). Assuming that CONFIG_SYS_SDRAM_SIZE is set to 128M in 
include/configs/rpi.h it should be always safe, but I will check that 
tomorrow to be 100% sure.


Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland