On 29/12/2021 04:04, miles.chen@mediatek.com wrote:
Hi,
On 28/12/2021 10:25, Miles Chen wrote:
Fix unused-but-set variable warning:
drivers/gpu/drm/mediatek/mtk_cec.c:85:6: warning: variable 'tmp' set but not used [-Wunused-but-set-variable]
Actually we ignore the value passed to mtk_cec_mask. In case of mtk_cec_mask(cec, CEC_CKGEN, 0 | CEC_32K_PDN, PDN | CEC_32K_PDN);
We are not setting CEC_32K_PDN. I wonder which side effect will it have to set that bit.
I am confused about "not setting CEC_32K_PDN" part, in case mtk_cec_mask(cec, CEC_CKGEN, 0 | CEC_32K_PDN, PDN | CEC_32K_PDN); CEC_32K_PDN (BIT(19)) is set.
for exmaple: CEC_32K_PDN is BIT(19) PDN is BIT(16) say tmp = 0xffffffff;
You mean reading the register returns 0xffffffff, correct?
mask = PDN | CEC_32K_PDN;
mask = 0x48000
val = 0 | CEC_32K_PDN;
val = 0x40000
tmp = fff6ffff, mask = 90000 val = 80000, tmp = fffeffff
u32 tmp = readl(cec->regs + offset) & ~mask; // tmp = fff6ffff
tmp = 0xffffffff & ~(0x48000) // tmp = 0xffb7ffff
tmp |= val & mask; // tmp = fffeffff
tmp |= 0x40000 & 0x48000 // tmp = 0xff7fffff
writel(val, cec->regs + offset); // val = 80000, tmp = fffeffff
val = 0x40000, tmp = 0xff7fffff
in both val and tmp case, CEC_32K_PDN is set.
You are right, in both cases the bit is set, but the funciton does not do what it is supposed to do.
With you fix: With the mask we define bits that a) are set to zero if not present in val b) set to one, when part of val, independent of what the value was in the register read.
mtk_cec_mask(cec, CEC_CKGEN, 0 | CEC_32K_PDN, PDN | CEC_32K_PDN);
Will set CEC_32K_PDN to one and clear PDN in the register.
mtk_cec_mask(cec, RX_GEN_WD, 0, HDMI_PORD_INT_32K_CLR | RX_INT_32K_CLR | HDMI_HTPLG_INT_32K_CLR | HDMI_PORD_INT_32K_EN | RX_INT_32K_EN | HDMI_HTPLG_INT_32K_EN);
Will just clear all bits of the mask.
Without your patch, we will just write the val to the register and don't care what the register value was before that.
We should somehow mention that in the commit message, as it's not only about a not used variable, it actually has an influence on the value we write(-back) to the register.
Regards, Matthias
Anyway, if it's the right thing to do, we should add:
Fixes: 8f83f26891e1 ("drm/mediatek: Add HDMI support")
I will add the Fixes tag, thanks.