linux-kernel - Re: [PATCH v2] dwc: dra7xx: Print link state to console for debug

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Message-ID: <53ed97ab-ed02-23be-0ce9-e79f2aaac979@ti.com>
Date:   Thu, 26 Oct 2017 13:29:27 +0530
From:   Faiz Abbas <faiz_abbas@...com>
To:     David Laight <David.Laight@...LAB.COM>,
        "kishon@...com" <kishon@...com>
CC:     "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] dwc: dra7xx: Print link state to console for debug

David,

On Thursday 19 October 2017 06:56 PM, David Laight wrote:
> From: Faiz Abbas
>> Sent: 19 October 2017 14:09
>> On Thursday 19 October 2017 06:13 PM, Faiz Abbas wrote:
>>> Enable support for printing the LTSSM link state for debugging PCI
>>> when link is down.
>>>
>>> Signed-off-by: Faiz Abbas <faiz_abbas@...com>
>>> ---
>>> v2:
>>>  1. Changed dev_err() to dev_dbg()
>>>  2. Changed static char array to static const char * const
>>>  3. format changes
>>>
>>>  drivers/pci/dwc/pci-dra7xx.c | 48 ++++++++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 48 insertions(+)
>>>
>>> diff --git a/drivers/pci/dwc/pci-dra7xx.c b/drivers/pci/dwc/pci-dra7xx.c
>>> index 34427a6..0e70e77 100644
>>> --- a/drivers/pci/dwc/pci-dra7xx.c
>>> +++ b/drivers/pci/dwc/pci-dra7xx.c
>>> @@ -98,6 +98,45 @@ struct dra7xx_pcie_of_data {
>>>
>>>  #define to_dra7xx_pcie(x)	dev_get_drvdata((x)->dev)
>>>
>>> +static const char * const state[] = {
>>> +	"DETECT_QUIET",
> ...
>>> +	"RCVRY_EQ3"
>>> +};
>>> +
>>>  static inline u32 dra7xx_pcie_readl(struct dra7xx_pcie *pcie, u32 offset)
>>>  {
>>>  	return readl(pcie->base + offset);
>>> @@ -118,6 +157,15 @@ static int dra7xx_pcie_link_up(struct dw_pcie *pci)
>>>  {
>>>  	struct dra7xx_pcie *dra7xx = to_dra7xx_pcie(pci);
>>>  	u32 reg = dra7xx_pcie_readl(dra7xx, PCIECTRL_DRA7XX_CONF_PHY_CS);
>>> +	u32 cmd_reg;
>>> +	u32 ltssm_state;
>>> +
>>> +	if (!(reg & LINK_UP)) {
>>> +		cmd_reg = dra7xx_pcie_readl(dra7xx,
>>> +					    PCIECTRL_DRA7XX_CONF_DEVICE_CMD);
>>> +		ltssm_state = (cmd_reg & GENMASK(7, 2)) >> 2;
>>> +		dev_dbg(pci->dev, "Link state:%s\n", state[ltssm_state]);
> 
> Hmmm... GENMASK leaves by hunting header files...> Why not (cmd_reg >> 2) & 63 and explicitly define state[64]
> to guarantee that you never print anything worse than a NULL
> pointer.

I'm not sure what you mean. Are you worried we might print something
outside the array bounds? How is this easier to decipher than GENMASK?

> 
>>> +	}
>>>
>>>  	return !!(reg & LINK_UP);
>>>  }
>>>
>>
>> I missed David's comment in v1. Will submit a new version. Please ignore.
> 
> I've a 'neat' trick for generating strings that match constants.
> You can get the compiler to do all the work for you:
> (Assuming I've typed it correctly)
> 
> #define LTSSM_DEFS(x) \
>   x(DETECT_QUIET) \
>   x(DETECT_ACT) \
> (continue for all the names)
> 
> Define an enum with the named constants:
> #define X(name) LTSSM_STATE_##name,
> enum (LTSSM_DEFS(X) LTSSM_STATE_SIZE=64);
> #undef X
> 
> Array of strings:
> #define X(name) [LTSSM_STATE_##name] = #name
> static const char * const state_names[LTSSM_STATE_SIZE] = { LTSSM_DEFS(X) };
> #undef X
> 
> 	David
> 

So I implemented your idea and it looks like this:
http://pastebin.ubuntu.com/25821834/

I don't know how much we gained by adding the trick. I still had to be
careful not to be off by 1 when writing the list. Plus we are never
saying anything like printk("%s", state[LTSSM_STATE_DETECT_QUIET]. Its a
register read which is used to index the list array.

Thanks,
Faiz