Investec has upgraded its rating for global banking giant HSBC from 'hold' to 'buy', saying that the downgrade cycle could finally be over.The broker said on Monday morning that despite the material improvement in credit experience seen in the third quarter and further encouraging trends in underlying cost performance, HSBC has continued to underperform UK domestic peers. Analyst Ian Gordon said: "Part of this is attributable to increased recognition of the revenue headwinds that the group faces, where sellside forecasts have persistently been much too high, as well as concern relating to outstanding US regulatory issues. But we suspect that, for HSBC, the downgrade cycle may finally be over."Investec said that HSBC's company-specific revenue headwinds have (historically) not been adequately reflected in sellside forecasts for net interest income (NII).The broker said that Bloomberg consensus expectations for full-year adjusted earnings per share (EPS) have fallen by 26% over the past 12 months. "We see this primarily as a reflection of the defective sellside forecasting of NII for the group," Gordon said.Investec has raised its target price for the stock from 590p to 630p. "From here, we expect outperformance versus Lloyds ('sell') and RBS ('sell')."BC