On 12/20/2024 3:16 PM, Jiri Slaby wrote:
On 19. 12. 24, 13:42, Zhongqiu Han wrote:
It is considered good practice to call cpu_relax() in busy loops, see
Documentation/process/volatile-considered-harmful.rst. This can lower CPU
power consumption or yield to a hyperthreaded twin processor, or serve as
a compiler barrier. In addition, if something goes wrong in the busy loop
at least it can prevent things from getting worse.
Signed-off-by: Zhongqiu Han <quic_zhonhan@xxxxxxxxxxx>
---
drivers/tty/mips_ejtag_fdc.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/mips_ejtag_fdc.c b/drivers/tty/mips_ejtag_fdc.c
index afbf7738c7c4..b17ead1e9698 100644
--- a/drivers/tty/mips_ejtag_fdc.c
+++ b/drivers/tty/mips_ejtag_fdc.c
@@ -346,7 +346,7 @@ static void mips_ejtag_fdc_console_write(struct
console *c, const char *s,
/* Busy wait until there's space in fifo */
while (__raw_readl(regs + REG_FDSTAT) & REG_FDSTAT_TXF)
- ;
+ cpu_relax();
__raw_writel(word.word, regs + REG_FDTX(c->index));
}
out:
@@ -1233,7 +1233,7 @@ static void kgdbfdc_push_one(void)
/* Busy wait until there's space in fifo */
while (__raw_readl(regs + REG_FDSTAT) & REG_FDSTAT_TXF)
- ;
+ cpu_relax();
Can this instead be switched to read_poll_timeout_atomic() or alike?
Those already contain cpu_relax(), of course...
thanks,
Hi Jiri,
Thanks a lot for the review!
yeah, maybe we can consider read_poll_timeout_atomic() or alike.
The implementation of read_poll_timeout_atomic() provides a precise
customization of the address busy read poll behavior by calling udelay()
and cpu_relax(), and using a timeout threshold. However, in this case,
it seems we might not need to customize the poll behavior. Since
udelay() only consumes CPU cycles, perhaps cpu_relax() is sufficient?
And if it times out, we still need to keep retrying until the data is
read. My initial thought was to call cpu_relax() to save power or act as
a memory barrier. As I mentioned before in my email to Greg, certain
MIPS-based architectures, such as Loongson-3, should requirecpu_relax().
Thanks~
--
Thx and BRs,
Zhongqiu Han
