Re: [PATCHv2] ata: pata_ep93xx: add missing "cc" clobber to inline asm

[Damien Le Moal <dlemoal@kernel.org>]

On 7/21/26 09:12, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [Critical] Trailing byte transfers in `ep93xx_pata_data_xfer()` silently corrupt data on big-endian systems due to incorrect endian swapping and variable truncation. For READ, the code assigns the 16-bit swapped result of `cpu_to_le16(ep93xx_pata_read_data(...))` to `*pad`, which is an 8-bit `unsigned char`. This truncates the 16-bit value. On big-endian systems, `cpu_to_le16()` swaps the bytes, so this truncation discards the lower byte (the correct data from the bus) and saves the upper byte to the buffer. For WRITE, `le16_to_cpu(*pad)` promotes the 8-bit byte to an integer before swapping it, resulting in a shifted 16-bit value, which causes the device to receive corrupted data on the wrong data lines.
> - [High] The inline assembly delay loop `ep93xx_pata_delay()` lacks a `"memory"` clobber, permitting the compiler to reorder it with respect to I/O accesses, which violates ATA PIO timing requirements. On older ARM architectures like ARMv4T (used in EP93xx) where `CONFIG_ARM_DMA_MEM_BUFFERABLE` is not selected, the standard I/O macros (`__iormb()` and `__iowmb()`) evaluate to empty, meaning `writel()` and `readl()` provide no compiler memory barriers at all. Because `__raw_readl()` and `__raw_writel()` are implemented using `asm volatile` without `"memory"` clobbers, GCC's instruction scheduler is completely free to reorder these volatile I/O accesses across the `ep93xx_pata_delay()` loop. Even if memory barriers were present after the read or before the write, the `asm volatile` instructions themselves lack dependencies and can be swapped.
> --
> 
> --- Patch [1]: [PATCHv2] ata: pata_ep93xx: add missing "cc" clobber to inline asm ---
> commit e8afdf774c87485ee82a0fe97b982fc47fc827bc
> Author: Rosen Penev <rosenp@gmail.com>
> 
> ata: pata_ep93xx: add missing "cc" clobber to inline asm
> 
> This commit adds a missing cc clobber to the inline assembly in
> ep93xx_pata_delay() to prevent the compiler from reordering flag-dependent
> instructions.
> 
>> diff --git a/drivers/ata/pata_ep93xx.c b/drivers/ata/pata_ep93xx.c
>> index 42a24dc51d26b..0bb748a86ac6e 100644
>> --- a/drivers/ata/pata_ep93xx.c
>> +++ b/drivers/ata/pata_ep93xx.c
>> @@ -211,6 +211,7 @@ static void ep93xx_pata_delay(unsigned long count)
>>  		"bge 0b\n"
>>  		: "=r" (count)
>>  		: "0" (count)
>> +		: "cc"
> 
> [Severity: High]
> This isn't a bug introduced by this patch, but since you are updating the
> clobbers in ep93xx_pata_delay(), should this inline assembly also include a
> "memory" clobber?

Rosen,

Can you look into this please and fix it in this same patch ?
The other critical problem can be fixed in a different patch.


> 
> On older ARM architectures like ARMv4T used in EP93xx, standard I/O macros
> can evaluate to empty, providing no compiler memory barriers. 
> 
> Without a memory clobber here, the compiler optimizer might be free to
> reorder independent I/O accesses across this delay loop, which could violate
> ATA PIO timing requirements.
> 
>>  	);
>>  #else
>>  	while (count--)
> 
> [ ... ]
> 
> [Severity: Critical]
> This is a pre-existing issue in the same file, but could the trailing byte
> transfers in ep93xx_pata_data_xfer() silently corrupt data on big-endian
> systems due to truncation?
> 
> For READ transfers, the code assigns a swapped value to an 8-bit variable:
> 
> drivers/ata/pata_ep93xx.c:ep93xx_pata_data_xfer() {
>     ...
>     *pad = cpu_to_le16(ep93xx_pata_read_data(drv_data, IDECTRL_ADDR_DATA));
>     ...
> }
> 
> Since pad is an unsigned char pointer, this truncates the 16-bit swapped
> result. On big-endian systems, cpu_to_le16() swaps the bytes, so truncating
> it to 8 bits discards the lower byte containing the correct data and saves
> the upper byte instead.
> 
> Similarly for WRITE transfers:
> 
> drivers/ata/pata_ep93xx.c:ep93xx_pata_data_xfer() {
>     ...
>     ep93xx_pata_write_data(drv_data, le16_to_cpu(*pad), IDECTRL_ADDR_DATA);
>     ...
> }
> 
> Promoting the 8-bit pad value to an integer before swapping it on a
> big-endian system seems to shift the byte to the wrong data lines, sending
> corrupted data to the device.
> 


-- 
Damien Le Moal
Western Digital Research