Re: [PATCH 3/4] amba: support masking data abort during identification

[Sascha Hauer <s.hauer@pengutronix.de>]

On Tue, Mar 18, 2025 at 09:25:06AM +0100, Ahmad Fatoum wrote:
> Registration of an AMBA device requires reading its pid and cid
> registers after turning on the device. This is the first I/O done to the
> device and may end up crashing if the device tree doesn't describe
> the correct clock and power domains or if a previous boot stage
> should've enabled them, but didn't.
> 
> This is not a likely issue, but when it happens it crashes the system
> annoyingly early during device tree populating.
> 
> Add a symbol that masks the data abort exception around the
> identification code to turn the crash into an error message instead.

Do we need to make this optional? I could imagine I find out about the
existence of this option only after having traced the issue down to the
exact lines of code in which case I won't need the option anymore.

Sascha

> 
> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> ---
>  drivers/amba/Kconfig | 19 +++++++++++++++++++
>  drivers/amba/bus.c   | 12 +++++++++++-
>  2 files changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/amba/Kconfig b/drivers/amba/Kconfig
> index 444d4ce7435c..be5e053f4b84 100644
> --- a/drivers/amba/Kconfig
> +++ b/drivers/amba/Kconfig
> @@ -1,3 +1,22 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  config ARM_AMBA
>  	bool
> +
> +config ARM_AMBA_DABT_MASK
> +	bool
> +	prompt "Mask data abort while identifying AMBA devices" if COMPILE_TEST
> +	depends on ARM_AMBA && ARCH_HAS_DATA_ABORT_MASK
> +	help
> +	  Registration of an AMBA device requires reading its pid and cid
> +	  registers after turning on the device, which can involve enabling
> +	  clocks, deasserting resets and powering on power domains.
> +	  In some cases, these resources are not available to barebox running
> +	  in the normal world and thus need to be setup by a previous
> +	  stage bootloader running in the secure world.
> +	  If such setup doesn't happen, accessing devices may lead to
> +	  a data abort or complete system hang.
> +
> +	  As a workaround for the former case, this option can be enabled
> +	  to gracefully handle data aborts during readout of the PID/CID
> +	  registers. The proper solution is fixing your first stage
> +	  bootloader or allow barebox access to the missing resources.
> diff --git a/drivers/amba/bus.c b/drivers/amba/bus.c
> index 6463366fafc5..5ed8336e6124 100644
> --- a/drivers/amba/bus.c
> +++ b/drivers/amba/bus.c
> @@ -10,6 +10,7 @@
>  #include <linux/amba/bus.h>
>  #include <io.h>
>  #include <init.h>
> +#include <abort.h>
>  
>  #define to_amba_driver(d)	container_of(d, struct amba_driver, drv)
>  
> @@ -132,6 +133,9 @@ int amba_device_add(struct amba_device *dev)
>  	if (ret == 0) {
>  		u32 pid, cid;
>  
> +		if (IS_ENABLED(CONFIG_ARM_AMBA_DABT_MASK))
> +			data_abort_mask();
> +
>  		/*
>  		 * Read pid and cid based on size of resource
>  		 * they are located at end of region
> @@ -139,8 +143,14 @@ int amba_device_add(struct amba_device *dev)
>  		pid = amba_device_get_pid(tmp, size);
>  		cid = amba_device_get_cid(tmp, size);
>  
> -		if (cid == AMBA_CID)
> +		if (IS_ENABLED(CONFIG_ARM_AMBA_DABT_MASK) && data_abort_unmask()) {
> +			dev_err(amba_bustype.dev,
> +				"data abort during MMIO read of PID/CID for %pOF\n",
> +				dev->dev.of_node);
> +			ret = -EFAULT;
> +		} else if (cid == AMBA_CID) {
>  			dev->periphid = pid;
> +		}
>  
>  		if (!dev->periphid)
>  			ret = -ENODEV;
> -- 
> 2.39.5
> 
> 
> 

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