Date:   Mon, 12 Feb 2018 10:51:52 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Jean Delvare <jdelvare@...e.de>
Cc:     Wolfram Sang <wsa@...-dreams.de>,
        Zoltán Böszörményi <zboszor@...hu>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] i2c: piix4: Use request_muxed_region

On Mon, Feb 12, 2018 at 11:10:41AM +0100, Jean Delvare wrote:
> Hi Guneter,
> 
> Sorry for the delay :(
> 
> On Sat, 30 Dec 2017 08:50:57 -0800, Guenter Roeck wrote:
> > Accesses to SB800_PIIX4_SMB_IDX can occur from multiple drivers.
> > Use request_muxed_region() to ensure synchronization.
> 
> Which ones? Documenting it, at least in the commit message, would seem
> useful. Out of curiosity, have these other drivers been converted to
> use request_muxed_region already?
> 
Primarily watchdog, but there is also unprotected initialization code
in several locations. I did convert the watchdog driver, and the changes
will be in v4.16. I did not touch the other code since none of the calls
has an error return.

> > 
> > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> > ---
> >  drivers/i2c/busses/i2c-piix4.c | 50 ++++++++++++++++++------------------------
> >  1 file changed, 21 insertions(+), 29 deletions(-)
> > 
> > diff --git a/drivers/i2c/busses/i2c-piix4.c b/drivers/i2c/busses/i2c-piix4.c
> > index 462948e2c535..78dd5951d6e7 100644
> > --- a/drivers/i2c/busses/i2c-piix4.c
> > +++ b/drivers/i2c/busses/i2c-piix4.c
> > @@ -153,10 +153,7 @@ static const struct dmi_system_id piix4_dmi_ibm[] = {
> >  
> >  /*
> >   * SB800 globals
> > - * piix4_mutex_sb800 protects piix4_port_sel_sb800 and the pair
> > - * of I/O ports at SB800_PIIX4_SMB_IDX.
> >   */
> > -static DEFINE_MUTEX(piix4_mutex_sb800);
> 
> With this gone, you can remove #include <linux/mutex.h>.
> 
> >  static u8 piix4_port_sel_sb800;
> >  static u8 piix4_port_mask_sb800;
> >  static u8 piix4_port_shift_sb800;
> > @@ -298,12 +295,15 @@ static int piix4_setup_sb800(struct pci_dev *PIIX4_dev,
> >  	else
> >  		smb_en = (aux) ? 0x28 : 0x2c;
> >  
> > -	mutex_lock(&piix4_mutex_sb800);
> > +	if (!request_muxed_region(SB800_PIIX4_SMB_IDX, 2, "sb800_piix4_smb"))
> > +		return -EBUSY;
> 
> This would happen if and only if another driver has requested the
> region already but without IORESOURCE_MUXED, right? Don't you want to

Or if its call to alloc_resource() fails.

> write an error message then? I don't think request_muxed_region() will
> do, and probe failing with -EBUSY but no error message logged would be
> hard to diagnose.
> 
NP, though the analysis is quite simple - /proc/iomem will show the culprit.

> > +
> >  	outb_p(smb_en, SB800_PIIX4_SMB_IDX);
> >  	smba_en_lo = inb_p(SB800_PIIX4_SMB_IDX + 1);
> >  	outb_p(smb_en + 1, SB800_PIIX4_SMB_IDX);
> >  	smba_en_hi = inb_p(SB800_PIIX4_SMB_IDX + 1);
> > -	mutex_unlock(&piix4_mutex_sb800);
> > +
> > +	release_region(SB800_PIIX4_SMB_IDX, 2);
> >  
> >  	if (!smb_en) {
> >  		smb_en_status = smba_en_lo & 0x10;
> > @@ -373,7 +373,12 @@ static int piix4_setup_sb800(struct pci_dev *PIIX4_dev,
> >  			break;
> >  		}
> >  	} else {
> > -		mutex_lock(&piix4_mutex_sb800);
> > +		if (!request_muxed_region(SB800_PIIX4_SMB_IDX, 2,
> > +					  "sb800_piix4_smb")) {
> > +			release_region(piix4_smba, SMBIOSIZE);
> > +			return -EBUSY;
> > +		}
> > +
> >  		outb_p(SB800_PIIX4_PORT_IDX_SEL, SB800_PIIX4_SMB_IDX);
> >  		port_sel = inb_p(SB800_PIIX4_SMB_IDX + 1);
> >  		piix4_port_sel_sb800 = (port_sel & 0x01) ?
> > @@ -381,7 +386,7 @@ static int piix4_setup_sb800(struct pci_dev *PIIX4_dev,
> >  				       SB800_PIIX4_PORT_IDX;
> >  		piix4_port_mask_sb800 = SB800_PIIX4_PORT_IDX_MASK;
> >  		piix4_port_shift_sb800 = SB800_PIIX4_PORT_IDX_SHIFT;
> > -		mutex_unlock(&piix4_mutex_sb800);
> > +		release_region(SB800_PIIX4_SMB_IDX, 2);
> >  	}
> >  
> >  	dev_info(&PIIX4_dev->dev,
> > @@ -679,7 +684,8 @@ static s32 piix4_access_sb800(struct i2c_adapter *adap, u16 addr,
> >  	u8 port;
> >  	int retval;
> >  
> > -	mutex_lock(&piix4_mutex_sb800);
> > +	if (!request_muxed_region(SB800_PIIX4_SMB_IDX, 2, "sb800_piix4_smb"))
> > +		return -EBUSY;
> 
> Did you check the performance cost? I thought that
> request_muxed_region() was meant for driver setup, I did not expect it
> to be used at driver run-time. Requesting the region again for every
> transaction seems quite costly?
> 
I did check why the driver has such a bad performance, which is why
I submitted the other patch to change msleep() to usleep_range().

Evaulating the actual per-call overhead seems to be quite pointless, unless
someone volunteers to introduce a specific access API for situations like this.
It is definitely not a unique situation - I have to do something similar
in the out-of-tree it87 driver, for example.

> That being said, being slow is certainly better than failing, as is
> currently the case, so I'm fine with this change anyway. Just curious.
> 
> >  
> >  	/* Request the SMBUS semaphore, avoid conflicts with the IMC */
> >  	smbslvcnt  = inb_p(SMBSLVCNT);
> > @@ -695,8 +701,8 @@ static s32 piix4_access_sb800(struct i2c_adapter *adap, u16 addr,
> >  	} while (--retries);
> >  	/* SMBus is still owned by the IMC, we give up */
> >  	if (!retries) {
> > -		mutex_unlock(&piix4_mutex_sb800);
> > -		return -EBUSY;
> > +		retval = -EBUSY;
> > +		goto release;
> >  	}
> >  
> >  	/*
> > @@ -753,8 +759,8 @@ static s32 piix4_access_sb800(struct i2c_adapter *adap, u16 addr,
> >  	if ((size == I2C_SMBUS_BLOCK_DATA) && adapdata->notify_imc)
> >  		piix4_imc_wakeup();
> >  
> > -	mutex_unlock(&piix4_mutex_sb800);
> > -
> > +release:
> > +	release_region(SB800_PIIX4_SMB_IDX, 2);
> >  	return retval;
> >  }
> >  
> > @@ -899,13 +905,6 @@ static int piix4_probe(struct pci_dev *dev, const struct pci_device_id *id)
> >  		bool notify_imc = false;
> >  		is_sb800 = true;
> >  
> > -		if (!request_region(SB800_PIIX4_SMB_IDX, 2, "smba_idx")) {
> > -			dev_err(&dev->dev,
> > -			"SMBus base address index region 0x%x already in use!\n",
> > -			SB800_PIIX4_SMB_IDX);
> > -			return -EBUSY;
> > -		}
> > -
> >  		if (dev->vendor == PCI_VENDOR_ID_AMD &&
> >  		    dev->device == PCI_DEVICE_ID_AMD_KERNCZ_SMBUS) {
> >  			u8 imc;
> > @@ -922,20 +921,16 @@ static int piix4_probe(struct pci_dev *dev, const struct pci_device_id *id)
> >  
> >  		/* base address location etc changed in SB800 */
> >  		retval = piix4_setup_sb800(dev, id, 0);
> > -		if (retval < 0) {
> > -			release_region(SB800_PIIX4_SMB_IDX, 2);
> > +		if (retval < 0)
> >  			return retval;
> > -		}
> >  
> >  		/*
> >  		 * Try to register multiplexed main SMBus adapter,
> >  		 * give up if we can't
> >  		 */
> >  		retval = piix4_add_adapters_sb800(dev, retval, notify_imc);
> > -		if (retval < 0) {
> > -			release_region(SB800_PIIX4_SMB_IDX, 2);
> > +		if (retval < 0)
> >  			return retval;
> > -		}
> >  	} else {
> >  		retval = piix4_setup(dev, id);
> >  		if (retval < 0)
> > @@ -983,11 +978,8 @@ static void piix4_adap_remove(struct i2c_adapter *adap)
> >  
> >  	if (adapdata->smba) {
> >  		i2c_del_adapter(adap);
> > -		if (adapdata->port == (0 << piix4_port_shift_sb800)) {
> > +		if (adapdata->port == (0 << piix4_port_shift_sb800))
> >  			release_region(adapdata->smba, SMBIOSIZE);
> > -			if (adapdata->sb800_main)
> > -				release_region(SB800_PIIX4_SMB_IDX, 2);
> > -		}
> >  		kfree(adapdata);
> >  		kfree(adap);
> >  	}
> 
> Everything else looks good to me, thanks.
> 
> I assume you have tested this patch on real hardware?
> 
I have been running the code on several systems since I submitted
the patch, together with the related changes in the watchdog driver.

Guenter

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