Boot hangs during sdhci_transfer_data_dma

[Sascha Hauer]

Hi Robin,

On Mon, Jun 20, 2022 at 04:33:02PM +0200, Robin van der Gracht wrote:
> Hi,
> 
> Today I tried to run barebox with CONFIG_KEYBOARD_GPIO=y added to my config.
> and noticed my board hangs during boot. When I modify the probe function to
> run without registering the poller[1] it boots as expected.
> 
> I started digging into the code to see how far the boot gets when I do
> register the poller. I found that Barebox hangs in a do/while loop in
> sdhci_transfer_data_dma[2].
> 
> The contents of the interrupt status (SDHCI_INT_STATUS) is 0 and stays that
> way forever trapping the process in the loop.
> 
> Call stack:
> 
> initcall -> barebox_of_populate
>   state_probe                                          drivers/misc/state.c
>     state_new_from_node                                common/state/state.c
>       of_find_path_by_node                             drivers/of/of_path.c
>         __of_find_path                                 drivers/of/of_path.c
>           device_detect                                drivers/base/driver.c
>             mci_detect_card                            drivers/mci/mci-core.c
>               mci_card_probe                           drivers/mci/mci-core.c
>                 mci_startup                            drivers/mci/mci-core.c
>                   mci_startup_mmc                      drivers/mci/mci-core.c
>                     mmc_compare_ext_csds               drivers/mci/mci-core.c
>                       mci_send_ext_csd                 drivers/mci/mci-core.c
>                         mci_send_cmd                   drivers/mci/mci-core.c
>                           esdhc_send_cmd
> drivers/mci/imx-esdhc.c
>                             __esdhc_send_cmd
> drivers/mci/imx-esdhc-common.c
>                               sdhci_transfer_data_dma  drivers/mci/sdhci.c
> 
> 
> I'm not sure how this happens. It's not the first transfer taking place. I
> figured that mayby the poller[1] just adds some cpu load that opens up a
> window for this to occur.
> 
> Maybe something else cleared the status register right before we entered the
> loop. Thats when I spotted this read/write construction[3]. It's executed
> right before we enter the do/while loop and (over)writes to the irq status
> register.
> 
> I removed the line with the write command[3] and my board boots as expected.
> Why are we (over)writing the status register right after reading it?

The idea is likely that we clear the interrupts that we just handled. It
seems by the time the status register is overwritten the DMA transfer is
already ongoing, and in your case even already done. We should only ever
clear the bits we have handled, like sdhci_transfer_data_dma() does with

	sdhci_write32(sdhci, SDHCI_INT_STATUS, SDHCI_INT_DMA);

Apart from this line the code never checks the same bit twice, so
clearing anything shoudn't be necessary. Clearing the status register
once either at the start or the end of the function should be enough.

I think the right thing to do is just to remove the erroneous status
register write like you already did.

Sascha

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