Re: [PATCH 0/6] RISC-V: Add eMMC support for TH1520 boards

[Drew Fustini <dfustini@baylibre.com>]

On Fri, Sep 22, 2023 at 05:48:21PM -0500, Robert Nelson wrote:
> On Fri, Sep 22, 2023 at 2:08 PM Robert Nelson <robertcnelson@gmail.com> wrote:
> >
> > On Thu, Sep 21, 2023 at 8:51 PM Drew Fustini <dfustini@baylibre.com> wrote:
> > >
> > > This series adds support for the eMMC on the BeagleV Ahead and the
> > > Sipeed LicheePi 4A. This allows the kernel to boot with the rootfs on
> > > eMMC.
> > >
> > > I tested on top of v6.6-rc2 with this config [1]. I was able to boot
> > > both the Ahead [2] and LPi4a [3] from eMMC. The following prerequisites
> > > are required:
> > >
> > >   [PATCH v2] riscv: dts: thead: set dma-noncoherent to soc bus [4]
> > >
> > > I pushed a branch [5] with this patch series and the above patch for
> > > those that find a git branch easier to test.
> > >
> > > Please note that only the MMC controller connected to the eMMC device
> > > is enabled in the device trees for these two boards. I did not yet
> > > attempt to configure and use the microSD card slot. My preference is to
> > > address that in a future patch series.
> > >
> > > References:
> > > [1] https://gist.github.com/pdp7/5fbdcf2a65eb1abdd3a29d519c19cdd2
> > > [2] https://gist.github.com/pdp7/91a801a5f8d1070c53509eda9800ad78
> > > [3] https://gist.github.com/pdp7/1445c3c991e88fd69c60165cef65726a
> > > [4] https://lore.kernel.org/linux-riscv/20230912072232.2455-1-jszhang@kernel.org/
> > > [5] https://github.com/pdp7/linux/tree/b4/th1520-mmc
> >
> > This patchset came out very nice!
> >
> > v6.6-rc2 with Last RFC v2:
> >
> > [    4.066630] mmc0: SDHCI controller on ffe7080000.mmc
> > [ffe7080000.mmc] using PIO
> >
> > debian@BeagleV:~$ sudo hdparm -tT /dev/mmcblk0
> >
> > /dev/mmcblk0:
> >  Timing cached reads:   1516 MB in  2.00 seconds = 758.09 MB/sec
> >  Timing buffered disk reads:  84 MB in  3.01 seconds =  27.94 MB/sec
> >
> > vs v6.6-rc2 with this patchset:
> >
> >  [    4.096837] mmc0: SDHCI controller on ffe7080000.mmc
> > [ffe7080000.mmc] using DMA
> >
> > debian@BeagleV:~$ sudo hdparm -tT /dev/mmcblk0
> >
> > /dev/mmcblk0:
> >  Timing cached reads:   1580 MB in  2.00 seconds = 790.97 MB/sec
> >  Timing buffered disk reads: 418 MB in  3.00 seconds = 139.11 MB/sec
> 
> Drew pointed out on Slack, this was not quite right.. After more
> digging by Drew, CONFIG_DMA_GLOBAL_POOL is causing a DMA limitation
> with the multiplatform defconfig. so with,
> 
> ./scripts/config --disable CONFIG_ARCH_R9A07G043
> 
> (to remove CONFIG_DMA_GLOBAL_POOL)... another 2x in buffered reads..
> 
> [    4.059242] mmc0: SDHCI controller on ffe7080000.mmc
> [ffe7080000.mmc] using ADMA 64-bit
> 
> debian@BeagleV:~$ sudo hdparm -tT /dev/mmcblk0
> 
> /dev/mmcblk0:
>  Timing cached reads:   1600 MB in  2.00 seconds = 800.93 MB/sec
>  Timing buffered disk reads: 892 MB in  3.00 seconds = 297.06 MB/sec

It seems CONFIG_DMA_GLOBAL_POOL=y causes ADMA buffer alloc to fail [1]:

  mmc0: Unable to allocate ADMA buffers - falling back to standard DMA

Prabhakar's AX45MP non-coherent DMA support [2] series introduced the
selection of DMA_GLOBAL_POOL for ARCH_R9A07G043 and the riscv defconfig
selects ARCH_R9A07G043. 

Patch 5 in the series [3] states that:

  With DMA_GLOBAL_POOL enabled all DMA allocations happen from this
  region and synchronization callbacks are implemented to synchronize
  when doing DMA transactions.

This example of a "shared-dma-pool" node was given:

        pma_resv0@58000000 {
            compatible = "shared-dma-pool";
            reg = <0x0 0x58000000 0x0 0x08000000>;
            no-map;
            linux,dma-default;
        };

I've copied that to th1520-beaglev-ahead.dts. The address of 0x58000000
has no significance on th1520, but the existence of shared-dma-pool
seems to fix the problem. ADMA mode [4] is now working even though
CONFIG_DMA_GLOBAL_POOL=y.

Thanks,
Drew

[1] https://gist.github.com/pdp7/73041ed808bbc7dd445836fb90574979
[2] https://lore.kernel.org/linux-riscv/20230818135723.80612-1-prabhakar.mahadev-lad.rj@bp.renesas.com/
[3] https://lore.kernel.org/linux-riscv/20230818135723.80612-6-prabhakar.mahadev-lad.rj@bp.renesas.com/
[4] https://gist.github.com/pdp7/91e72a663d3bb73eb28182337ad8bbcb