Re: [PATCH v2] soc: qcom: Rework BCM_TCS_CMD macro

[Stephen Boyd <sboyd@kernel.org>]

Quoting Eugen Hristev (2024-10-29 06:12:12)
> On 10/28/24 19:56, Stephen Boyd wrote:
> > Quoting Eugen Hristev (2024-10-28 09:34:03)
> >> diff --git a/include/soc/qcom/tcs.h b/include/soc/qcom/tcs.h
> >> index 3acca067c72b..152947a922c0 100644
> >> --- a/include/soc/qcom/tcs.h
> >> +++ b/include/soc/qcom/tcs.h
[....]
> >>   /* Construct a Bus Clock Manager (BCM) specific TCS command */
> >>   #define BCM_TCS_CMD(commit, valid, vote_x, vote_y)             \
> >> -       (((commit) << BCM_TCS_CMD_COMMIT_SHFT) |                \
> >> -       ((valid) << BCM_TCS_CMD_VALID_SHFT) |                   \
> >> -       ((cpu_to_le32(vote_x) &                                 \
> >> -       BCM_TCS_CMD_VOTE_MASK) << BCM_TCS_CMD_VOTE_X_SHFT) |    \
> >> -       ((cpu_to_le32(vote_y) &                                 \
> >> -       BCM_TCS_CMD_VOTE_MASK) << BCM_TCS_CMD_VOTE_Y_SHFT))
> >> +       (le32_encode_bits(commit, BCM_TCS_CMD_COMMIT_MASK) |    \
> >> +       le32_encode_bits(valid, BCM_TCS_CMD_VALID_MASK) |       \
> >> +       le32_encode_bits(vote_x,        \
> >> +                       BCM_TCS_CMD_VOTE_X_MASK) |              \
> >> +       le32_encode_bits(vote_y,        \
> >> +                       BCM_TCS_CMD_VOTE_Y_MASK))
> > 
> > Why is cpu_to_le32() inside BCM_TCS_CMD at all? Is struct tcs_cmd::data
> > supposed to be marked as __le32?
> > 
> > Can the whole u32 be constructed and turned into an __le32 after setting
> > all the bit fields instead of using le32_encode_bits() multiple times?
> 
> I believe no. The fields inside the constructed TCS command should be 
> little endian. If we construct the whole u32 and then convert it from 
> cpu endinaness to little endian, this might prove to be incorrect as it 
> would swap the bytes at the u32 level, while originally, the bytes for 
> each field that was longer than 1 byte were swapped before being added 
> to the constructed u32.
> So I would say that the fields inside the constructed item are indeed 
> le32, but the result as a whole is an u32 which would be sent to the 
> hardware using an u32 container , and no byte swapping should be done 
> there, as the masks already place the fields at the required offsets.
> So the tcs_cmd.data is not really a le32, at least my acception of it.
> Does this make sense ?
> 

Sort of? But I thought that the RPMh hardware was basically 32-bit
little-endian registers. That's why write_tcs_*() APIs in
drivers/soc/qcom/rpmh-rsc.c use writel() and readl(), right? The
cpu_to_le32() code that's there today is doing nothing, because the CPU
is little-endian 99% of the time. It's likely doing the wrong thing on
big-endian machines. Looking at commit 6311b6521bcc ("drivers: qcom: Add
BCM vote macro to header") it seems to have picked the macro version
from interconnect vs. clk subsystem. And commit b5d2f741077a
("interconnect: qcom: Add sdm845 interconnect provider driver") used
cpu_to_le32() but I can't figure out why.

If the rpmh-rsc code didn't use writel() or readl() I'd believe that the
data member is simply a u32 container. But those writel() and readl()
functions are doing a byte swap, which seems to imply that the data
member is a native CPU endian u32 that needs to be converted to
little-endian. Sounds like BCM_TCS_CMD() should just pack things into a
u32 and we can simply remove the cpu_to_l32() stuff in the macro?