On 12/19/2024 9:28 PM, Greg KH wrote:
On Thu, Dec 19, 2024 at 08:42:54PM +0800, 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();
How did you test this? Are you _sure_ it is needed at all? I think you
just made these loops take a lot longer than before :(
Have you had problems with these tight loops doing anything bad to your
system?
thanks,
greg k-h
Hi Greg,
Thanks a lot for the review~
Perhaps I should submit an RFC patch and explain the situation, as I
don't have a MIPS device for testing. Indeed, the cpu_relax()
implementation for MIPS is a memory barrier, which, compared to busy
waiting, doesn't save power and can make loops slower than before.
However, according to its definition file, for certain MIPS-based
architectures like Loongarch-3, it can help force the Loongson-3 SFB
(Store-Fill-Buffer) flush to avoid pending writes. Below is the
implementation of cpu_relax() for the MIPS architecture and its
comments.
-----------------------------------------------------------------
arch/mips/include/asm/vdso/processor.h
#ifdef CONFIG_CPU_LOONGSON64
/*
* Loongson-3's SFB (Store-Fill-Buffer) may buffer writes indefinitely
* when a tight read loop is executed, because reads take priority over
* writes & the hardware (incorrectly) doesn't ensure that writes will
* eventually occur.
*
* Since spin loops of any kind should have a cpu_relax() in them, force
* an SFB flush from cpu_relax() such that any pending writes will
* become visible as expected.
*/
#define cpu_relax() smp_mb()
#else
#define cpu_relax() barrier()
#endif
----------------------------------------------------------------
Based on this, cpu_relax() should be needed here? :)
Thank you~
--
Thx and BRs,
Zhongqiu Han
