On Mon, Dec 22, 2025 at 11:24:38AM +0800, Bibo Mao wrote:
>
>
> On 2025/12/21 下午8:22, Yao Zi wrote:
> > IOCSRs could be accessed in any sizes from 1 to 8 bytes as long as the
> > address is aligned, regardless whether through MMIO or iocsr{rd,wr}
> > instructions. Lower min_access_size to 1 byte for IOCSR memory region to
> > match real-hardware behavior.
> Hi Yao,
>
> What is the detailed problem you encountered? Or just look through code and
> think that it should be so.
I don't think there's a real use-case for this. However, without the
patch, the behavior of iocsrrd.b differs between real hardware and QEMU,
you could try this diff with Linux kernel for comparing.
diff --git a/arch/loongarch/kernel/setup.c b/arch/loongarch/kernel/setup.c
index 25a87378e48e..679e311ac654 100644
--- a/arch/loongarch/kernel/setup.c
+++ b/arch/loongarch/kernel/setup.c
@@ -621,3 +621,13 @@ void __init setup_arch(char **cmdline_p)
kasan_init();
#endif
}
+
+static int __init read_iocsr(void)
+{
+ pr_info("%s: iocsrrd_b(0x10) = 0x%x\n", __func__,
+ __iocsrrd_b(0x10));
+
+ return 0;
+}
+
+late_initcall(read_iocsr);
On QEMU, the error raised by address_space_ldub is silently ignored by
helper_iocsrrd_b(), and thus __iocsrrd_b(0x10) results in zero, while on
real hardware it doesn't.
Ignoring the error returned by address_space_{ld,st}* in helper_iocsr*
could cause more behaviors inconsistent with real LoongArch hardware.
But in the case shown in the diff that a single byte is read from iocsr,
the access shouldn't fail at all, which this patch tries to fix.
Regards,
Yao Zi
> IOCSR supports 1/2/4/8 byte access like MMIO, however here is IOCSR MISC
> device rather than IOCSR bus emulation. What is the usage and scenery to
> read IOCSR MISC device with one byte?
>
> It is similar with other device emulation with MMIO, such as e1000e with 4
> bytes aligned rather than any byte:
> static const MemoryRegionOps mmio_ops = {
> .read = e1000e_mmio_read,
> .write = e1000e_mmio_write,
> .endianness = DEVICE_LITTLE_ENDIAN,
> .impl = {
> .min_access_size = 4,
> .max_access_size = 4,
> },
> };
>
>
> Regards
> Bibo Mao
