linux-kernel - Re: [Intel-wired-lan] [PATCH] driver core: fix e1000e ltr bug

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Date:   Wed, 30 Jun 2021 09:13:36 +0300
From:   "Neftin, Sasha" <sasha.neftin@...el.com>
To:     Yee Li <seven.yi.lee@...il.com>
Cc:     jesse.brandeburg@...el.com, anthony.l.nguyen@...el.com,
        davem@...emloft.net, kuba@...nel.org,
        intel-wired-lan@...ts.osuosl.org, linux-kernel@...r.kernel.org,
        "Ruinskiy, Dima" <dima.ruinskiy@...el.com>,
        "Edri, Michael" <michael.edri@...el.com>,
        "Efrati, Nir" <nir.efrati@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH] driver core: fix e1000e ltr bug

On 6/29/2021 20:33, Yee Li wrote:
> 
> Yes, 18874368ns > 3145728ns.
> But, 0xe40 < 0x1003.
I got you. I would agree, direct comparison is error-prone. (10M is 
impacted)
I would suggest do not use convert function. lat_en should rather 
presented as lat_enc = scale x value:
Introduce two u16 variables, u16 lat_enc_d and u16 max_ltr_enc_d;

lat_enc_d = (lat_enc & 0x0x3ff) x (1U << 5*((max_ltr_enc & 0x1c00) >> 10))

max_ltr_enc_d = (max_ltr_enc & 0x0x3ff) x (1U << 5*((max_ltr_enc & 
0x1c00) >> 10))

Then:
if (lat_enc_d > max_ltr_enc_d)
	lat_enc = max_ltr_enc;
what do you think?

> 
> So, the final lat_enc is 0xe40.
> (Latency encoded is less than maximum LTR encoded by platform)
> 
> Neftin, Sasha <sasha.neftin@...el.com <mailto:sasha.neftin@...el.com>> 
> 于 2021年6月29日周二 22:49写道：
> 
>     On 6/29/2021 11:21, YeeLi wrote:
>     Yeeli,
>      > In e1000e driver, a PCIe-like device, the max snoop/no-snoop latency
>      > is the upper limit.So, directly compare the size of lat_enc and
>      > max_ltr_enc is incorrect.
>      >
>     why?
>      >      In 1000Mbps, 0x8b9 < 0x1003, 189440 ns < 3145728 ns, correct.
>      >
>      >      In 100Mbps, 0xc3a < 0x1003, 1900544 ns < 3145728 ns, correct.
>      >
>      >      In 10Mbps, 0xe40 < 0x1003, 18874368 > 3145728, incorrect.
>      >
>     Platform LTR encoded is 0x1003 - right. It is meant 1048576ns x 3 =
>     3145738ns.
>     Now,
>     for 1000M: 0x08b9 => 185ns x 1024 = 189440ns (you are correct)
>     for 100M: 0x0c3a => 58ns x 32768 = 1900544ns (correct)
>     for 10M: 0x0e41 => 577ns x 32768 = 18907136ns (ok?)
>     18907136ns > 3145738ns, (latency encoded is great than maximum LTR
>     encoded by platform) - so, there is no point to wait more than platform
>     required, and lat_enc=max_ltr_enc. It is expected and we sent right
>     value to the power management controller.
>     What is the problem you try solve?
> 
>      > Decoded the lat_enc and max_ltr_enc before compare them is necessary.
>      >
>      > Signed-off-by: YeeLi <seven.yi.lee@...il.com
>     <mailto:seven.yi.lee@...il.com>>
>      > ---
>      >   drivers/net/ethernet/intel/e1000e/ich8lan.c | 23
>     ++++++++++++++++++++-
>      >   1 file changed, 22 insertions(+), 1 deletion(-)
>      >
>      > diff --git a/drivers/net/ethernet/intel/e1000e/ich8lan.c
>     b/drivers/net/ethernet/intel/e1000e/ich8lan.c
>      > index 590ad110d383..3bff1b570b76 100644
>      > --- a/drivers/net/ethernet/intel/e1000e/ich8lan.c
>      > +++ b/drivers/net/ethernet/intel/e1000e/ich8lan.c
>      > @@ -986,6 +986,27 @@ static s32 e1000_k1_workaround_lpt_lp(struct
>     e1000_hw *hw, bool link)
>      >       return ret_val;
>      >   }
>      >
>      > +static u32 convert_e1000e_ltr_scale(u32 val)
>      > +{
>      > +     if (val > 5)
>      > +             return 0;
>      > +
>      > +     return 1U << (5 * val);
>      > +}
>      > +
>      > +static u64 decoded_ltr(u32 val)
>      > +{
>      > +     u64 decoded_latency;
>      > +     u32 value;
>      > +     u32 scale;
>      > +
>      > +     value = val & 0x03FF;
>      > +     scale = (val & 0x1C00) >> 10;
>      > +     decoded_latency = value * convert_e1000e_ltr_scale(scale);
>      > +
>      > +     return decoded_latency;
>      > +}
>      > +
>      >   /**
>      >    *  e1000_platform_pm_pch_lpt - Set platform power management
>     values
>      >    *  @hw: pointer to the HW structure
>      > @@ -1059,7 +1080,7 @@ static s32 e1000_platform_pm_pch_lpt(struct
>     e1000_hw *hw, bool link)
>      >                                    E1000_PCI_LTR_CAP_LPT + 2,
>     &max_nosnoop);
>      >               max_ltr_enc = max_t(u16, max_snoop, max_nosnoop);
>      >
>      > -             if (lat_enc > max_ltr_enc)
>      > +             if (decoded_ltr(lat_enc) > decoded_ltr(max_ltr_enc))
>      >                       lat_enc = max_ltr_enc;
>      >       }
>      >
>      >
>     sasha
>