Message-ID: <cd2729ee-203a-ea59-9ad5-843a39caa2b5@schinagl.nl>
Date:   Wed, 29 Mar 2017 19:20:18 +0200
From:   Olliver Schinagl <o.schinagl@...imaker.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Jason Uy <jason.uy@...adcom.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Ed Blake <ed.blake@...tec.com>, linux-serial@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dev@...ux-sunxi.org
Subject: Re: [PATCH] serial: 8250_dw: Minor code cleanup

Hey Doug,

On 29-03-17 19:10, Olliver Schinagl wrote:
> Hey Doug,
>
> On 29-03-17 17:50, Doug Anderson wrote:
>> Hi,
>>
>> On Wed, Mar 29, 2017 at 3:04 AM, Olliver Schinagl <oliver@...inagl.nl>
>> wrote:
>>> Commit 424d79183af0 ("serial: 8250_dw: Avoid "too much work" from
>>> bogus rx timeout interrupt")
>>> added a bit check with quite a wide mask. To be concise with the other
>>> similar calls in this driver, change it to mask against the flag we
>>> want to
>>> check only. This thus removes a magic value/mask.
>>
>> How certain are you that your patch is correct?  You are now basically
>> checking to see if the bits "0xc" are set in the IIR.  Previously the
>> patch ensured that the bits 0x33 were clear.
>
> You raise a good point. And after writing two replies that made perfect
> sense, I just realized looking at the table you wisely posted below, I
> should have spotted that the interrupts are not bits! So very good catch
> and my bad indeed.
>
>>
>> Have you tried looking through the kernel for other places where
>> UART_IIR_RX_TIMEOUT is referenced?  In 8250_omap.c and 8250_port.c I
>> believe you'll find it masking against 0x3f.  In omap-serial.c you'll
>> see a mask against 0x3e.
I just checked the 8250_omap case and the reason they mask against 0x3f 
is because they have 2 extra bits in the iir, e.g. the 'reserved' bits 
are not reserved there but used. So either 8250_omap is not based on the 
same designware IP or they added it ontop of the DW ip.

And thus, would the define be UART_IIR_MASK or UART_IIR_DW_MASK and 
UART_IIR_OMAP_MASK? Or do we, as mentioned earlier, rely on the fact 
that it is supposed to be reserved but always read as 0?

The 0x3e case seems to be where the busy flag is to be always ignored, 
so UART_IIR_NOBUSY_MASK? But also kind of ugly, as this is not a 
bitfield irq! so it should be changed there to read:

UART_IIR_BUSY: /* fallthrough */
default:
	break;

right?

>>
>> Looking at the TRM for rk3399, I see that bits 4 and 5 (bitmask 0x30)
>> as "reserved".  I see the following definitions for bits 3:0:
>>
>> 0000 = modem status
>> 0001 = no interrupt pending
>> 0010 = THR empty
>> 0100 = received data available
>> 0110 = receiver line status
>> 0111 = busy detect
>> 1100 = character timeout
>>
>> ...so while your patch will probably function OK, it would also
>> function equally well to simply test bit 3 (0x80) and ignore
>> everything else.  ...but IMHO it is more correct to at least mask the
>> IIR with 0x0F and confirm that bits 2 and 3 are set and bits 0 and 1
>> are zero.  ...and since the main 8250 code uses 0x3f, that seems even
>> better to me (despite the fact that it seems to be relying on the fact
>> that the "reserved" bits come back as 0).
>
> I strongly agree with you here, I did it wrong, but 0x3f really is wrong
> too imo. The bits to look at are 3:0, bits 4:5 are reserved and we
> should never look at those, as as you rightfully put it are being relied
> on to be 0 (which may always be the case) but imo is still wrong and
> thus the mask should be 0x0f.
>
> Going to the horse's mouth [0] which is the documentation of the IP
> block used in all these designs, they also say the same thing. 4 bits
> and while I don't have any of the other datasheets of other 8250 cores,
> I bet they are the same?
>
> And then, the following is actually wrong on the same grounds, from the
> 8250_dw.c
>
>
> if ((iir & UART_IIR_BUSY) == UART_IIR_BUSY) {
>
> This just happens to work as it is the only way this can match, but
> clearly it is wrong then, right?
>
> I'll check against the same mask of 0x0f here as well.
>>
>>
>> If you want to make a fix, I'd suggest adding a #define for 0x3f and
>> using it in various places.
> Yeah I'll do that, add the define next to the others for 0x0f, unless
> someone says it is a good idea (or needed idea) to mask against the
> reserved bits as well.
>
>>
>>
>>> Some very minor code cleanups, such as including the bitops header for
>>> DW_UART_MCR_SIRE, use the BIT() macro as suggested by checkpatc and
>>> removed a whitespace to match other invocations.
>>
>> Maybe it's just me, but it seems like a bad idea to combine these
>> cleanups in the same patch with a functional change..
> Well with the oversight of the bit check above, it becomes obvious that
> it is a bigger change indeed. I will change it!
>
> Olliver
>
>>
>
> [0] http://linux-sunxi.org/images/d/d2/Dw_apb_uart_db.pdf

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